stm32/Makefile: Use -Wno-attributes for ll_usb.c HAL source file.
A recent version of arm-none-eabi-gcc (8.2.0) will warn about unused packed
attributes in USB_WritePacket and USB_ReadPacket.  This patch suppresses
such warnings for this file only.
@@ -303,6 +303,7 @@ SRC_HAL = $(addprefix $(HAL_DIR)/Src/stm32$(MCU_SERIES)xx_,\
)
ifeq ($(MCU_SERIES),$(filter $(MCU_SERIES),f4 f7 h7 l4))
+$(BUILD)/$(HAL_DIR)/Src/stm32$(MCU_SERIES)xx_ll_usb.o: CFLAGS += -Wno-attributes
SRC_HAL += $(addprefix $(HAL_DIR)/Src/stm32$(MCU_SERIES)xx_,\
hal_sd.c \
ll_sdmmc.c \
