feat(build): Build unified config/mod repos.

* Detect in our build script if our config repo is *also* a Zephyr
  module and if so pass to ZMK build in ZMK_EXTRA_MODULES
  define.

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

@@ -59,8 +59,9 @@ jobs:
           shield: ${{ matrix.shield }}
           artifact_name: ${{ matrix.artifact-name }}
         run: |
+          if [ -e zephyr/module.yml ]; then export zmk_extra_modules_arg=" -DZMK_EXTRA_MODULES='$(pwd)'"; fi
           echo "zephyr_version=${ZEPHYR_VERSION}" >> $GITHUB_ENV
-          echo "extra_cmake_args=${shield:+-DSHIELD=\"$shield\"}" >> $GITHUB_ENV
+          echo "extra_cmake_args=${shield:+-DSHIELD=\"$shield\"}${zmk_extra_modules_arg}" >> $GITHUB_ENV
           echo "display_name=${shield:+$shield - }${board}" >> $GITHUB_ENV
           echo "artifact_name=${artifact_name:-${shield:+$shield-}${board}-zmk}" >> $GITHUB_ENV