openocd: src: replace the incomplete GPL-2.0-or-later license tag

openocd: src: replace the incomplete GPL-2.0-or-later license tag

Few files have the FSF boilerplate without the latest statement on
where to get the GPL license.

Manually replace the FSF boilerplate with the SPDX tag.
While there, reorganize the copyright statement.

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: I0c908d01c010e24f9c7e94885e7fbed4ecf26a86
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7063
Tested-by: jenkins