Create GitHub workflow for CI

.github/workflows/build.yml

@@ -0,0 +1,17 @@
+name: CI
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@master
+      - name: Install deps
+        run: sudo apt install gcc-arm-none-eabi binutils-arm-none-eabi
+      - name: Compile
+        run: sh build.sh