add board to workflow

Author: hierophect

.github/workflows/build.yml

@@ -215,6 +215,7 @@ jobs:
         - "stm32f411ce_blackpill"
         - "stm32f411ve_discovery"
         - "stm32f412zg_discovery"
+        - "stm32f4_discovery"
         - "stringcar_m0_express"
         - "teensy40"
         - "teknikio_bluebird"