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.

0 files changed, 0 insertions, 0 deletions