boards/arduino-mega2560: Use code style CFLAGS from Makefile.cflags

boards/arduino-mega2560/Makefile.include

@@ -44,8 +44,8 @@ endif
 
 # define build specific options
 export CPU_USAGE = -mmcu=atmega2560
-export CFLAGS += -ggdb -g3 -std=gnu99 -Os -Wall -Wstrict-prototypes $(CPU_USAGE)
+export CFLAGS += -ggdb -g3 -Os $(CPU_USAGE)
 export ASFLAGS += -ggdb -g3 $(CPU_USAGE) $(FPU_USAGE)
-export LINKFLAGS += -g3 -ggdb -std=gnu99 $(CPU_USAGE) $(FPU_USAGE) -static -lgcc -e reset_handler
+export LINKFLAGS += -g3 -ggdb $(CPU_USAGE) $(FPU_USAGE) -static -lgcc -e reset_handler
 export OFLAGS += -j .text -j .data -O ihex
 export FFLAGS += -p m2560 -c $(PROGRAMMER) $(PROGRAMMER_FLAGS) -F -U flash:w:bin/$(BOARD)/$(PROJECT)$(APPLICATION).hex