Implement GitHub Action to compile firmware image

.github/workflows/main.yml

@@ -0,0 +1,25 @@
+name: CI
+
+on: [push]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Cache downloads
+      uses: actions/cache@v1
+      with:
+        path: dl/
+        key: dl
+    - name: Update submodules
+      run: git submodule update --init
+    - name: Compile
+      run: |
+        cd buildroot
+        make BR2_EXTERNAL=$PWD/.. thirtythreeforty_businesscard_defconfig
+        make
+    - uses: actions/upload-artifact@v1
+      with:
+        name: flash.bin
+        path: buildroot/output/images/flash.bin

README.md

@@ -1,5 +1,7 @@
 # Business Card Linux
 
+[![Actions Status](https://github.com/thirtythreeforty/businesscard-linux/workflows/CI/badge.svg)](https://github.com/thirtythreeforty/businesscard-linux/actions)
+
 This is the Linux distribution small enough to run from the 8MB of storage on my Linux-powered business card.
 It is powered by the Allwinner F1C100s, a $1.40 Linux-capable ARM system-on-chip.
 This repository is the source code for its firmware image, based on Buildroot 2019.05.