fix iar make build with wb and u5

hw/bsp/stm32u5/boards/stm32u5a5nucleo/board.mk

@@ -1,3 +1,4 @@
+MCU_VARIANT = stm32u5a5xx
 CFLAGS += \
   -DSTM32U5A5xx \
   -DHSE_VALUE=16000000UL \
@@ -5,8 +6,5 @@ CFLAGS += \
 # All source paths should be relative to the top level.
 LD_FILE = ${BOARD_PATH}/STM32U5A5ZJTXQ_FLASH.ld
 
-SRC_S += $(ST_CMSIS)/Source/Templates/gcc/startup_stm32u5a5xx.s
-
-MCU_VARIANT = stm32u5a5xx
 # For flash-jlink target
 JLINK_DEVICE = stm32u575zi