gitlab-ci: check all defconfigs on every push

gitlab-ci: check all defconfigs on every push

The defconfig check has been introduced by the previous
patch before the building each defconfig but those builds
are done every week or more.

Checking if a defconfig is valid can be done on every
push in the repository since it take few seconds.

This would allow to detect as soon as possible a problem
in a defconfig and eventually avoid breaking the build
while build testing all defconfig.

Introduce a new job template ".defconfig_check" in
gitlab-ci.yml.in and modify the generate-gitlab-ci-yml
to create a job for each defconfig to run the test.

Although, we could have used only one job to do all
tests, using one job per defconfig allow to identify
easily in gitlab which defconfig is falling.

Tested:
https://gitlab.com/kubu93/buildroot/pipelines/138331069
https://gitlab.com/kubu93/buildroot/pipelines/171223758

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>