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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>
4 jobs for patches/child-pipeline/v2 in 206 minutes and 29 seconds (queued for 8 seconds)
Child pipeline (parent)
Status Job ID Name Coverage
  Build
passed #951180121
build-4.4:board-beagle-bone-black

03:03:21

passed #951180122
build-4.4:board-x86-64-efi

02:11:53

 
  Test
passed #951180123
lava-test-4.4:board-beagle-bone-black

00:23:08

passed #951180124
lava-test-4.4:board-x86-64-efi

00:22:38