stm32/Makefile: Allow a board's .mk file to add things to CFLAGS.
@@ -87,7 +87,7 @@ CFLAGS_MCU_l4 = $(CFLAGS_CORTEX_M) -mtune=cortex-m4 -mcpu=cortex-m4
CFLAGS_MCU_h7 = $(CFLAGS_CORTEX_M) -mtune=cortex-m7 -mcpu=cortex-m7
CFLAGS_MCU_wb = $(CFLAGS_CORTEX_M) -mtune=cortex-m4 -mcpu=cortex-m4
-CFLAGS = $(INC) -Wall -Wpointer-arith -Werror -std=gnu99 -nostdlib $(CFLAGS_MOD) $(CFLAGS_EXTRA)
+CFLAGS += $(INC) -Wall -Wpointer-arith -Werror -std=gnu99 -nostdlib $(CFLAGS_MOD) $(CFLAGS_EXTRA)
CFLAGS += -D$(CMSIS_MCU)
CFLAGS += $(CFLAGS_MCU_$(MCU_SERIES))
CFLAGS += $(COPT)
