boards/cortexm_common: Use code style CFLAGS from Makefile.cflags

cpu/Makefile.include.cortexm_common

@@ -35,17 +35,16 @@ ifneq (llvm,$(TOOLCHAIN))
 # not building with LLVM
 export CFLAGS_CPU  += -mno-thumb-interwork
 endif
-export CFLAGS_STYLE = -std=gnu99 -Wall -Wstrict-prototypes -Werror=implicit-function-declaration
 export CFLAGS_LINK  = -ffunction-sections -fdata-sections -fno-builtin -fshort-enums
 export CFLAGS_DBG   = -ggdb -g3
 export CFLAGS_OPT  ?= -Os
 
-export CFLAGS += $(CFLAGS_CPU) $(CFLAGS_STYLE) $(CFLAGS_LINK) $(CFLAGS_DBG) $(CFLAGS_OPT)
+export CFLAGS += $(CFLAGS_CPU) $(CFLAGS_LINK) $(CFLAGS_DBG) $(CFLAGS_OPT)
 
 export ASFLAGS += $(CFLAGS_CPU) $(CFLAGS_DEBUG)
 export LINKFLAGS += -L$(RIOTCPU)/$(CPU)/ldscripts -L$(RIOTCPU)/cortexm_common/ldscripts
 export LINKFLAGS += -T$(RIOTCPU)/$(CPU)/ldscripts/$(CPU_MODEL).ld -Wl,--fatal-warnings
-export LINKFLAGS += $(CFLAGS_DEBUG) $(CFLAGS_CPU) $(CFLAGS_STYLE) -static -lgcc -nostartfiles
+export LINKFLAGS += $(CFLAGS_DEBUG) $(CFLAGS_CPU) -static -lgcc -nostartfiles
 export LINKFLAGS += -Wl,--gc-sections
 
 # This CPU implementation is using the new core/CPU interface: