ath25: mark as source-only

ath25: mark as source-only

ath25 seems to be a target with low number of users according to download
statistics, most of which are for older releases anyway.
Users that we managed to find are currently building images downstream as
due to low amount of RAM (32MB) default config will not work.

Target also suffers from inability for the 5.15 kernel bump to be tested
which is a requirment for the next release.

So, for those reasons, lets mark it as source-only so that Buildbots dont
use the resources for building the images for this target anymore.

Signed-off-by: Robert Marko <robimarko@gmail.com>