Add SPDX identifier to liscense in "new" file

Author: theotherjimmy

tools/toolchains/mbed_toolchain.py

@@ -1,5 +1,6 @@
 """
 mbed SDK
+SPDX-License-Identifier: Apache-2.0
 Copyright (c) 2011-2013 ARM Limited
 
 Licensed under the Apache License, Version 2.0 (the "License");