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.

1 files changed, 8 insertions, 5 deletions

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,