ci: Introduce parent-child pipelines

After adding the necessary builds for xenomai-3.1,
we have 16 jobs in each stage. This many jobs decrease
the clarity of the ci pipeline. This commit splits the
pipeline into the following child pipelines[1]:
- xenomai 3.0.x with kernel 4.4
- xenomai 3.1.x with kernel 4.19
- xenomai next with kernel 4.19 and kernel 5.4

[1]: https://docs.gitlab.com/ee/ci/parent_child_pipelines.html

Signed-off-by: Quirin Gylstorff <quirin.gylstorff@siemens.com>
[Jan: fixed new lines at EOF]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
3 jobs for next in 0 seconds (queued for 101 minutes and 34 seconds)
Status Job ID Name Coverage
  Parent
passed xenomai 3.0.x

passed xenomai 3.1.x

passed xenomai next