feat: Add ZMK_BOARD_COMPAT Kconfig symbol, and check for it
To properly detect users selecting a base board ID, when a ZMK variant
is available, add a ZMK_BOARD_COMPAT Kconfig symbol that the ZMK
variants will enable, allowing us to detect using the bare varand and
warn/error in CI so users don't end up with passing CI that results in
an unusable firmware due to missing ZMK enablement for keyboard name,
settings storage, etc.
refactor(boards): Add ZMK_BOARD_COMPAT to ZMK board variants
To clearly indicate the ZMK board variants are set up for ZMK usage,
select the ZMK_BOARD_COMPAT symbols for them.
fix(boards): Fix pillbug board.yml to report extension
Properly not the ZMK variant is part of an extension of the existing
pillbug board.
