Add gitlab-CI support

Add gitlab-CI support

The buildroot repository is now mirrored on
https://gitlab.com/buildroot.org/buildroot so we can use Gitlab-CI to
test Buildroot. Gitlab-CI is controlled by a .gitlab-ci.yml file
that exists in the repository.

For now, the only test is building all defconfigs (inspired on
https://travis-ci.org/buildroot/buildroot-defconfig-testing/). Since
all the defconfigs have to be specified in the .gitlab-ci.yml file,
we generate the file based on .gitlab-ci.yml.in. The generated
.gitlab-ci.yml file has to be committed into the repository, though,
otherwise Gitlab-CI doesn't see it. So there is also a test to verify
that .gitlab-ci.yml is up-to-date.

Building all the defconfigs takes a long time. Gitlab-CI will do that
every time it pulls from git.buildroot.org, which is once per hour.
That is way too often. Therefore, the defconfigs are not built on pull,
but only on explicit trigger through the API or when a tag is added.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas:
 - fix typo not -> no
 - add LC_ALL=C when calling 'ls -1' to get a predictable order of the
   defconfigs
 - regenerate .gitlab-ci.yml.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>