contrib: replace the GPLv3-or-later license tag

These files are not built in OpenOCD and are not in conflict with
OpenOCD license.

Replace the GPL boilerplate with the SPDX tag.
Add the SPDX tag.

The SPDX tag on files *.c is incorrect, as it should use the C99
single line comment using '//'. But current checkpatch doesn't
allow C99 comments, so keep using standard C comments, by now.

Change-Id: I934e4add24098f0e8574120b3bc32e9bcf166abd
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7157
Tested-by: jenkins

contrib/rpc_examples/ocd_rpc_example.py

@@ -1,4 +1,6 @@
 #!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 """
 OpenOCD RPC example, covered by GNU GPLv3 or later
 Copyright (C) 2014 Andreas Ortmann (ortmann@finf.uni-hannover.de)