introduce github actions

.github/workflows/main.yml

@@ -0,0 +1,16 @@
+name: Build
+
+on: [pull_request, push]
+
+jobs: 
+  build:
+    container:
+      image: ghcr.io/armmbed/mbed-os-env:latest
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Compile Starter Project
+        run: make
+        working-directory: 'apps/beats-time/make'
+

make.mk

@@ -20,7 +20,8 @@ else
   MKDIR = mkdir
 endif
 
-CFLAGS += -W -Wall --std=gnu99 -Os
+CFLAGS += -W -Wall -Wextra -Wmissing-prototypes -Wmissing-declarations
+CFLAGS += --std=gnu99 -Os
 CFLAGS += -fno-diagnostics-show-caret
 CFLAGS += -fdata-sections -ffunction-sections
 CFLAGS += -funsigned-char -funsigned-bitfields