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.

.github/workflows/build-user-config.yml

@@ -147,6 +147,24 @@ jobs:
           echo "See also the [list of released versions](https://github.com/zmkfirmware/zmk/releases)." >> $GITHUB_STEP_SUMMARY
           echo "If you wish to stay on main, check the most recent pending release PR for breaking changes. [Our blog](https://zmk.dev/blog) may have upgrade information if breaking changes are significant." >> $GITHUB_STEP_SUMMARY
 
+      - name: Check if building a board without explicit ZMK compat
+        if: always()
+        working-directory: ${{ env.base_dir }}/zmk/app
+        run: |
+          if ! (grep "CONFIG_ZMK_BOARD_COMPAT=y" "${{ env.build_dir }}/zephyr/.config" > /dev/null)
+          then
+            original_board=$(echo "${{ matrix.board }}" | sed -e 's$/.*$$')
+            west boards --board-root module --board-root . --board "${original_board}" --format "{qualifiers}" | grep "zmk" > /dev/null
+
+            if [ $? -ne 0 ]
+            then
+              echo "::warning file=build/zephyr/.config,title=Missing ZMK Compat::The selected board does not report explicit ZMK compat. Please verify you've selected the correct board and ZMK variant if one exists"
+            else
+              echo "::error file=build/zephyr/.config,title=Missing ZMK Compat::The selected board is not set up for ZMK and there is a ZMK variant available. See https://zmk.dev/blog/2025/12/09/zephyr-4-1#zmk-board-variant."
+              exit 1
+            fi
+          fi
+
       - name: ${{ env.display_name }} Kconfig file
         run: |
           if [ -f "${{ env.build_dir }}/zephyr/.config" ]