gitlab: generate the gitlab-ci configuration before each build

gitlab: generate the gitlab-ci configuration before each build

Since Gitlab 12.9, Gitlab allow to trigger child pipeline with generated configuration file.
See: https://gitlab.com/gitlab-org/gitlab/-/issues/35632

This allow us to stop updating the .gitlab-ci.yml file when a
new defconfig is added to Buildroot.

Remove check-gitlab-ci.yml job since it is now uneeded.
Remove .gitlab-ci.yml make target.

Signed-off-by: Romain Naour <[email protected]>
Cc: Arnout Vandecappelle <[email protected]>
[[email protected]: manual: no longer needed to update at all]
Signed-off-by: Yann E. MORIN <[email protected]>