Added the ability to specify a custom startup file for CMSIS (#582)

Author: Medan-rfz

builder/frameworks/cmsis.py

@@ -131,5 +131,10 @@ def prepare_startup_file(src_path):
     src_filter=[
         "-<*>",
         "+<%s>" % board.get("build.cmsis.system_file", "system_%sxx.c" % mcu[0:7]),
-        "+<gcc/startup_%s.S>" % product_line.lower()]
+        "+<gcc/%s>"
+        % board.get(
+            "build.cmsis.startup_file",
+            "startup_%s.S" % product_line.lower()
+        )
+    ]
 )