contrib: replace the GPLv2-or-later license tag

contrib: replace the GPLv2-or-later license tag

Replace the GPLv2-or-later boilerplate with 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: I380d552940f1c405309a3346454251c0e80b5a45
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7159
Tested-by: jenkins