hil: express a board's always-on defines as a variant, dropping build.args

The roster had two ways to pass a cmake -D to a board's build: `build.args`,
applied to every variant, and `variant[].defines`, applied to one. They did the
same thing, and only metro_m4_express used the first - for MAX3421_HOST=1, which
is what makes it the one rig board that compiles hcd_max3421.c.

A board whose define is always on now carries a single variant named after itself,
which is exactly the shape `board.get('variant') or [{'name': name, 'flags': ''}]`
already synthesises everywhere - so the build dir, the HIL report row and the
variant-boundary handling are unchanged. raspberry_pi_pico has used that shape
for its flags all along.

Removes the BuildCfg type and the parallel code path from all four consumers:
hil_test.build_board, hil_pool_check's two builders, hil_ci_set_matrix and
ci_select.board_options.

Verified: the hil-build matrix entry is byte-identical
(`-b metro_m4_express -DMAX3421_HOST=1`), hil_test's build command is unchanged,
ci_select still selects the board for a max3421 diff with MAX3421_HOST in its
options, and a real build of dual/host_info_to_device_cdc and host/cdc_msc_hid on
that board still compiles hcd_max3421.c.

6 files changed, 24 insertions, 29 deletions

diff --git a/.github/scripts/hil_ci_set_matrix.py b/.github/scripts/hil_ci_set_matrix.py
index 396c4175a..bf50061dd 100644
--- a/.github/scripts/hil_ci_set_matrix.py
+++ b/.github/scripts/hil_ci_set_matrix.py
@@ -103,8 +103,6 @@ def main():
                     f'hil-build-esp jobs in .github/workflows/build.yml')
 
             build_board = f'-b {name}'
-            if 'build' in board and 'args' in board['build']:
-                build_board += ' ' + ' '.join(f'-D{a}' for a in board['build']['args'])
 
             # PR selection: build only the examples this board will run (its test
             # list plus device/board_test, the parking firmware) - tools/build.py -e.
diff --git a/test/hil/helper/hil_pool_check.py b/test/hil/helper/hil_pool_check.py
index d926bbe3d..179a417ed 100644
--- a/test/hil/helper/hil_pool_check.py
+++ b/test/hil/helper/hil_pool_check.py
@@ -433,7 +433,7 @@ def build_example(board: dict, variant: str, example: str) -> int:
         cmd = ['idf.py', '-C', f'examples/{example}',
                '-B', f'cmake-build/cmake-build-{vcfg["name"]}/{example}',
                '-G', 'Ninja', f'-DBOARD={name}', 'build']
-        for d in board.get('build', {}).get('args', []) + vcfg.get('defines', []):
+        for d in vcfg.get('defines', []):
             cmd.insert(-1, f'-D{d}')
         if vcfg.get('flags'):
             cmd.insert(-1, f'-DCFLAGS_CLI={vcfg["flags"]}')
@@ -446,8 +446,6 @@ def build_example(board: dict, variant: str, example: str) -> int:
     cmd = [sys.executable, str(hil_util.TINYUSB_ROOT / 'tools' / 'build.py'),
            '-b', name, '-T', Path(example).name,
            '-j', str(max(1, (os.cpu_count() or _jobs) // _jobs))]
-    for d in board.get('build', {}).get('args', []):
-        cmd += ['-D', d]
     if vcfg['name'] != name:
         cmd += ['--build-name', vcfg['name']]
     for d in vcfg.get('defines', []):
diff --git a/test/hil/hil_test.py b/test/hil/hil_test.py
index 174251343..fcd7c7e6f 100755
--- a/test/hil/hil_test.py
+++ b/test/hil/hil_test.py
@@ -194,10 +194,6 @@ class TestsCfg(TypedDict, total=False):
     dev_attached: list[AttachedDevCfg]
 
 
-class BuildCfg(TypedDict, total=False):
-    args: list[str]
-
-
 class VariantCfg(TypedDict, total=False):
     name: str           # build dir (cmake-build-<name>) and HIL report row
     flags: str          # raw CFLAGS, e.g. "-DCFG_TUD_DWC2_DMA_ENABLE=1"
@@ -209,7 +205,9 @@ class Board(TypedDict):
     uid: str
     tests: TestsCfg
     flasher: FlasherCfg
-    build: NotRequired[BuildCfg]
+    # every build knob lives here, including a board's always-on defines: a board that
+    # needs one carries a single variant named after itself (metro_m4_express /
+    # MAX3421_HOST=1), which is exactly what the `or [...]` default below synthesises
     variant: NotRequired[list[VariantCfg]]
     toolchain: NotRequired[str]  # CI build bucket override, e.g. "riscv-gcc" (consumed by hil_ci_set_matrix.py)
 
@@ -1670,21 +1668,17 @@ def test_example(board: Board, variant: str, example: str) -> tuple[int, str, st
 
 def build_board(board: Board) -> tuple[str, int]:
     """Build firmware for this board via tools/build.py.
-    Honors board config's variant list and build.args defines.
+    Honors board config's variant list (name, defines, flags).
     Output goes to cmake-build/cmake-build-<variant>/ (tools/build.py layout).
 
     Unbounded on purpose: --build is a local convenience (no CI workflow passes it), so
     the developer watching the build is the timeout."""
     name = board['name']
-    bcfg = cast(BuildCfg, board.get('build', {}))
-    extra_defs = bcfg.get('args', [])
     variants = board.get('variant') or [{'name': name, 'flags': ''}]
 
     failed = 0
     for v in variants:
         cmd = [sys.executable, str(hil_util.TINYUSB_ROOT / 'tools' / 'build.py'), '-b', name]
-        for d in extra_defs:
-            cmd += ['-D', d]
         if v['name'] != name:
             cmd += ['--build-name', v['name']]
         for d in v.get('defines', []):
diff --git a/test/hil/test/test_ci_select.py b/test/hil/test/test_ci_select.py
index 031e8e287..f5c64ac1c 100644
--- a/test/hil/test/test_ci_select.py
+++ b/test/hil/test/test_ci_select.py
@@ -329,7 +329,7 @@ class TestOptionGatedPort(unittest.TestCase):
     # host-side option board (max3421 as host controller), off any max3421 family
     OPT_ROSTER = [('test/hil/opt.json', [
         {'name': 'fake_dual_board', 'uid': 'o1', 'flasher': {'name': 'jlink'},
-         'build': {'args': ['MAX3421_HOST=1']},
+         'variant': [{'name': 'fake_dual_board', 'defines': ['MAX3421_HOST=1']}],
          'tests': {'device': True, 'host': False, 'dual': True}},
         {'name': 'fake_host_board', 'uid': 'o2', 'flasher': {'name': 'jlink'},
          'variant': [{'name': 'fake_host_board', 'flags': '-DMAX3421_HOST=1'}],
@@ -346,10 +346,10 @@ class TestOptionGatedPort(unittest.TestCase):
         for board in boards:
             self.assertIn(board, s['boards'])
 
-    def test_option_selects_via_args_defines_and_flags(self):
+    def test_option_selects_via_defines_and_flags(self):
         s = ci_select.classify(['src/portable/analog/max3421/hcd_max3421.c'], REPO, self.OPT_ROSTER)
         self.assertFalse(s['full'])
-        self.assertIn('fake_dual_board', s['boards'])    # build.args
+        self.assertIn('fake_dual_board', s['boards'])    # variant defines
         self.assertIn('fake_host_board', s['boards'])    # variant flags
         self.assertNotIn('fake_off_board', s['boards'])  # variant defines, but =0
 
@@ -1762,7 +1762,7 @@ class TestBuildPyExampleFilter(unittest.TestCase):
                          {'tinyusb_metrics', 'cdc_msc', 'cdc_msc-membrowse-upload'})
 
     def test_build_defines_reach_the_example_filter(self):
-        # metro_m4_express gets MAX3421_HOST=1 from the roster build args, never
+        # metro_m4_express gets MAX3421_HOST=1 from its roster variant, never
         # from its BSP: without threading them through, -e drops the rig's only
         # MAX3421 dual firmware that --target all used to build
         self.assertIsNone(self.build.resolve_example_target_groups(
@@ -1934,7 +1934,7 @@ class TestSkipExampleMirrorsFamilyFilter(unittest.TestCase):
     def test_build_define_enables_max3421_only_list(self):
         # family_support.cmake:940 appends MAX3421 to FAMILY_MCUS when
         # MAX3421_HOST=1; on metro_m4_express that define comes from the roster
-        # build args, so skip_example has to be told about it
+        # variant defines, so skip_example has to be told about it
         ex = 'dual/host_info_to_device_cdc'
         self.assertTrue(self.build_utils.skip_example(ex, 'metro_m4_express'))
         self.assertFalse(self.build_utils.skip_example(ex, 'metro_m4_express',
diff --git a/test/hil/tinyusb.json b/test/hil/tinyusb.json
index 6f552f126..8fd4683a4 100644
--- a/test/hil/tinyusb.json
+++ b/test/hil/tinyusb.json
@@ -157,11 +157,14 @@
         {
             "name": "metro_m4_express",
             "uid": "9995AD485337433231202020FF100A34",
-            "build": {
-                "args": [
-                    "MAX3421_HOST=1"
-                ]
-            },
+            "variant": [
+                {
+                    "name": "metro_m4_express",
+                    "defines": [
+                        "MAX3421_HOST=1"
+                    ]
+                }
+            ],
             "tests": {
                 "device": true,
                 "host": false,
diff --git a/tools/ci_select.py b/tools/ci_select.py
index cd63899c1..ced3bbbc0 100755
--- a/tools/ci_select.py
+++ b/tools/ci_select.py
@@ -188,10 +188,12 @@ def bsp_board_options(board_name: str, repo_root: str) -> frozenset:
 
 
 def board_options(board: dict, repo_root: str) -> set:
-    """Build options a board has truthy: the roster entry's build.args plus each
-    variant's defines (NAME=VALUE) and raw CFLAGS (-DNAME=VALUE), plus whatever its
-    own board.cmake sets (a board can enable a gated port without the roster saying so)."""
-    toks = list(board.get('build', {}).get('args', []))
+    """Build options a board has truthy: each variant's defines (NAME=VALUE) and raw
+    CFLAGS (-DNAME=VALUE), plus whatever its own board.cmake sets (a board can enable a
+    gated port without the roster saying so). A board whose option is always on carries
+    a single variant named after itself - metro_m4_express and MAX3421_HOST=1, which is
+    what makes it the one rig board that compiles hcd_max3421.c."""
+    toks = []
     for v in board.get('variant', []):
         toks += list(v.get('defines', []))
         toks += v.get('flags', '').split()