buildroot

.gitlab-ci.yml: extend check-package test to Config.* files

Now that all issues in Config.in files have been fixed, let's try to
make sure we don't introduce new ones by checking regularly these files
issues in Gitlab CI.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>