gitlab-ci: only check defconfigs for known branches

gitlab-ci: only check defconfigs for known branches

Currently, the check of defconfigs is run for all branches, even those
that are pushed only to run runtime tests. This is very inconvenient.

In fact, we only want to check the defconfigs on standard branches, that
is master, next, and the maintenance branches.

This will also decrease drastically the number gitlab-ci minutes used
when one pushes their repo to gitlab.com, where the number of CI minutes
are now going to be pretty severely restricted.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>