stm32/mboot: Use STARTUP_FILE from stm32lib.

ports/stm32/mboot/Makefile

@@ -20,8 +20,10 @@ endif
 include ../../../py/mkenv.mk
 include $(BOARD_DIR)/mpconfigboard.mk
 
-CMSIS_DIR=$(TOP)/lib/stm32lib/CMSIS/STM32$(MCU_SERIES_UPPER)xx/Include
 MCU_SERIES_UPPER = $(shell echo $(MCU_SERIES) | tr '[:lower:]' '[:upper:]')
+CMSIS_MCU_LOWER = $(shell echo $(CMSIS_MCU) | tr '[:upper:]' '[:lower:]')
+
+CMSIS_DIR=$(TOP)/lib/stm32lib/CMSIS/STM32$(MCU_SERIES_UPPER)xx/Include
 HAL_DIR=lib/stm32lib/STM32$(MCU_SERIES_UPPER)xx_HAL_Driver
 USBDEV_DIR=usbdev
 DFU=$(TOP)/tools/dfu.py
@@ -30,6 +32,7 @@ DEVICE=0483:df11
 STFLASH ?= st-flash
 OPENOCD ?= openocd
 OPENOCD_CONFIG ?= boards/openocd_stm32f4.cfg
+STARTUP_FILE ?= lib/stm32lib/CMSIS/STM32$(MCU_SERIES_UPPER)xx/Source/Templates/gcc/startup_$(CMSIS_MCU_LOWER).o
 
 CROSS_COMPILE = arm-none-eabi-
 
@@ -101,7 +104,7 @@ SRC_C = \
 	$(wildcard $(BOARD_DIR)/*.c)
 
 SRC_O = \
-	ports/stm32/boards/startup_stm32$(MCU_SERIES).o \
+	$(STARTUP_FILE) \
 	ports/stm32/resethandler.o \
 
 $(BUILD)/$(HAL_DIR)/Src/stm32$(MCU_SERIES)xx_ll_usb.o: CFLAGS += -Wno-attributes