checkpatch: add variable $OpenOCD

checkpatch: add variable $OpenOCD

The script checkpatch.pl require some adaptation for OpenOCD that
cannot be achieved through the config file .checkpatch.conf; the
script's code has to be modified.
To merge new version of the script from Linux kernel it becomes
relevant highlighting the changes, while minimizing the diff wrt
the initial script.

Add the perl variable '$OpenOCD' and suggest how to highlight
changes.

Change-Id: Ia8d26426850008f0465858a1d84cc774bc1146ed
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7021
Tested-by: jenkins