CI: Add GitHub Actions workflow to build and upload hex file. (#737)

* CI: Add GitHub Actions workflow to build and upload hex file.

* ci: Add date to the uploaded artifact.

.github/workflows/build.yml

@@ -0,0 +1,47 @@
+name: Build MicroPython
+
+on:
+  push:
+    branches: '*'
+  pull_request:
+    branches: '*'
+
+jobs:
+  build:
+    runs-on: ubuntu-20.04
+    name: Build MicroPython
+    steps:
+      - uses: actions/checkout@v2
+      # Yotta has some issues with Python 3.7+
+      - name: Install Python 3.6
+        uses: actions/setup-python@v3
+        with:
+          python-version: 3.6
+      - name: Install GNU Arm Embedded Toolchain (arm-none-eabi-gcc)
+        uses: carlosperate/arm-none-eabi-gcc-action@v1
+        with:
+          release: "10.3-2021.10"
+      - name: Install CMake, Ninja & Yotta
+        run: pip install cmake==3.22.3 ninja==1.10.2.3 yotta==0.20.5
+      - name: Install srecord
+        run: sudo apt-get install -y srecord
+      - name: Check Versions
+        run: |
+          arm-none-eabi-gcc --version
+          cmake --version
+          ninja --version
+          python --version
+          yotta --version
+      - name: Set up the Yotta target from GitHub
+        run: |
+          yotta target bbc-microbit-classic-gcc-nosd@https://github.com/lancaster-university/yotta-target-bbc-microbit-classic-gcc-nosd
+          yotta up
+      - run: make all
+      - name: Process date for artifact filename
+        id: date
+        run: echo "::set-output name=date::$(date +'%Y-%m-%d')"
+      - name: Upload hex file
+        uses: actions/upload-artifact@v1
+        with:
+          name: microbitv1-micropython-${{ steps.date.outputs.date }}-${{ github.sha }}.hex
+          path: build/firmware.hex