stm32/mboot/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.
@@ -85,6 +85,7 @@ SRC_O = \
ports/stm32/boards/startup_stm32$(MCU_SERIES).o \
ports/stm32/resethandler.o \
+$(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_cortex.c \
hal_flash.c \
