gdb: add source-highlight PACKAGECONFIG option

gdb: add source-highlight PACKAGECONFIG option

>From gdb documentation:

> There are two ways that highlighting can be done.  First, if
> gdb was linked with the GNU Source Highlight library, then it
> is used.  Otherwise, if gdb was configured with Python
> scripting support, and if the Python Pygments package is available,
> then it will be used.

Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>