CI: add simple build workflow

.github/workflows/build.yml

@@ -0,0 +1,39 @@
+# This is a basic workflow to help you get started with Actions
+
+name: build
+
+# Controls when the action will run.
+on:
+  # Triggers the workflow on push or pull request events but only for the main branch
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+  # Allows you to run this workflow manually from the Actions tab
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v7
+
+      - name: Install meson, ninja
+        run: |
+          sudo apt-get update
+          sudo apt-get install --no-install-recommends -y meson ninja-build
+
+      - name: configure
+        run: |
+          meson setup --cross-file=llvm.ini --buildtype release -Dprefix=/usr _build
+
+      - name: Build
+        run: ninja -C _build
+
+      - uses: actions/upload-artifact@v7
+        with:
+          name: avd-firmware
+          path: |
+            _build/apple