buildroot

.gitlab-ci.yml: fix it again

It seems the ! is removed by the yaml parser, so it needs to be quoted.
While we're at it, add some explanatory comment.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>