Add ZMK_BOARD_COMPAT symbol, that we can enable for our ZMK variants (#3265)

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.

app/Kconfig

@@ -5,6 +5,17 @@ mainmenu "ZMK Firmware"
 
 menu "ZMK"
 
+config ZMK_BOARD_COMPAT
+    bool
+    help
+      Hidden symbol used to hint that a board has been specifically
+      set up with various ZMK requirements in mind, e.g. settings
+      storage, bootloader integration, etc.
+
+      Failure to set this will warn users they may want to verify they
+      used a ZMK variant of an upstream board, or are using a board
+      properly migrated to a newer ZMK version.
+
 menu "Basic Keyboard Setup"
 
 config ZMK_KEYBOARD_NAME