Fix cmsis startup link for L4 series

cmake/FindCMSIS.cmake

@@ -99,7 +99,10 @@ ELSEIF(STM32_FAMILY STREQUAL "L4")
 
     LIST(APPEND CMSIS_COMMON_HEADERS core_cm4.h)
     SET(CMSIS_DEVICE_HEADERS stm32l4xx.h system_stm32l4xx.h)
-    SET(CMSIS_DEVICE_SOURCES system_stm32l4xx.c)    
+    SET(CMSIS_DEVICE_SOURCES system_stm32l4xx.c) 
+    IF(NOT CMSIS_STARTUP_SOURCE)
+        SET(CMSIS_STARTUP_SOURCE startup_stm32l${STM32_CHIP_TYPE_LOWER}.s)
+    ENDIF()   
 ENDIF()
 
 IF(NOT CMSIS_STARTUP_SOURCE)