gitlab-ci: convert only/except to rules

gitlab-ci: convert only/except to rules

only/except keywords will be deprecated by upcoming gitlab release,
upstream recommend to use rules keyword instead [1][2][3][4][5].

This patch convert .gitlab-ci.yml to use rules, no functional
changes intended.

After this patch, we should still have the following behaviour
while pushing commit, tag, branches:

     - to trigger only the check-* jobs:
       $ git push gitlab HEAD:<name>

     - to trigger all defconfigs and all check-* jobs:
       $ git push gitlab HEAD:<name>-defconfigs

     - to trigger all runtime tests and all check-* jobs:
       $ git push gitlab HEAD:<name>-runtime-tests

     - to trigger one defconfig job:
       $ git push gitlab HEAD:<name>-<defconfig name>

     - to trigger one runtime job:
       $ git push gitlab HEAD:<name>-<test case name>

[1] https://gitlab.com/gitlab-org/gitlab/-/commit/7eaaa597e24bf24743bdd8f8d1d51ade83b3f6de
[2] https://about.gitlab.com/releases/2020/06/22/gitlab-13-1-released/#templates-to-simplify-initial-rules-keyword-configuration
[3] https://about.gitlab.com/releases/2020/05/22/gitlab-13-0-released/#auto-devops-and-secure-configuration-templates-are-changing-to-%60rules%60-instead-of-%60only/except%60
[4] https://gitlab.com/gitlab-org/gitlab/issues/27449
[5] https://gitlab.com/groups/gitlab-org/-/epics/2783
[6] https://buildroot.org/downloads/manual/manual.html#_using_the_run_tests_framework

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>