openocd: fix syntax of SPDX tags

openocd: fix syntax of SPDX tags

Put the SPDX tag alone in a comment in the first line of the file.
Replace the obsolete GPL-2.0+ 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: Ia91b0f7da42c439b6340bbe81983b86b68f6d65c
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7156
Tested-by: jenkins