openocd: src: replace the GPL with eCos exception 2.0 license tag

openocd: src: replace the GPL with eCos exception 2.0 license tag

Add the license exception text in the license pool.
Add the exception chapter in license-rules.txt
Replace the 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: Ied513b7c9c0722ed2a9c11dbdff3fbf59f1b41ce
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7061
Tested-by: jenkins