stm32/Makefile: Use startup_stm32*.s file from stm32lib.

This means that each MCU now gets a unique IRQ table, eg a specific one for
STM32F405, STM32F411, etc rather than just STM32F4xx.

ports/stm32/Makefile

@@ -24,9 +24,11 @@ FROZEN_MPY_DIR ?= modules
 # include py core make definitions
 include $(TOP)/py/py.mk
 
+MCU_SERIES_UPPER = $(shell echo $(MCU_SERIES) | tr '[:lower:]' '[:upper:]')
+CMSIS_MCU_LOWER = $(shell echo $(CMSIS_MCU) | tr '[:upper:]' '[:lower:]')
+
 LD_DIR=boards
 CMSIS_DIR=$(TOP)/lib/stm32lib/CMSIS/STM32$(MCU_SERIES_UPPER)xx/Include
-MCU_SERIES_UPPER = $(shell echo $(MCU_SERIES) | tr '[:lower:]' '[:upper:]')
 HAL_DIR=lib/stm32lib/STM32$(MCU_SERIES_UPPER)xx_HAL_Driver
 USBDEV_DIR=usbdev
 #USBHOST_DIR=usbhost
@@ -40,7 +42,7 @@ DEVICE=0483:df11
 STFLASH ?= st-flash
 OPENOCD ?= openocd
 OPENOCD_CONFIG ?= boards/openocd_stm32f4.cfg
-STARTUP_FILE ?= boards/startup_stm32$(MCU_SERIES).o
+STARTUP_FILE ?= lib/stm32lib/CMSIS/STM32$(MCU_SERIES_UPPER)xx/Source/Templates/gcc/startup_$(CMSIS_MCU_LOWER).o
 
 # Select the cross compile prefix
 CROSS_COMPILE ?= arm-none-eabi-
@@ -576,7 +578,6 @@ GEN_PLLFREQTABLE_HDR = $(HEADER_BUILD)/pllfreqtable.h
 GEN_STMCONST_HDR = $(HEADER_BUILD)/modstm_const.h
 GEN_STMCONST_QSTR = $(BUILD)/modstm_qstr.h
 GEN_STMCONST_MPZ = $(HEADER_BUILD)/modstm_mpz.h
-CMSIS_MCU_LOWER = $(shell echo $(CMSIS_MCU) | tr '[:upper:]' '[:lower:]')
 CMSIS_MCU_HDR = $(CMSIS_DIR)/$(CMSIS_MCU_LOWER).h
 
 modmachine.c: $(GEN_PLLFREQTABLE_HDR)