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>
12 jobs for next in 205 minutes and 53 seconds (queued for 34 seconds)
Child pipeline (parent)
Status Job ID Name Coverage
  Build
passed #199205
build-4.19:board-beagle-bone-black

01:11:10

passed #199203
build-4.19:board-hikey

01:33:46

passed #199196
build-4.19:board-qemu-amd64

01:34:39

passed #199201
build-4.19:board-qemu-arm64

01:32:02

passed #199198
build-4.19:board-qemu-armhf

01:27:10

passed #199207
build-4.19:board-x86-64-efi

01:23:16

 
  Test
passed #199222
lava-test-4.19:board-beagle-bone-black

00:10:24

passed #199220
lava-test-4.19:board-hikey

00:20:30

passed #199224
lava-test-4.19:board-x86-64-efi

00:23:19

passed #199210
lava-test-4.19:qemu-amd64

01:50:54

passed #199217
lava-test-4.19:qemu-arm64

01:34:50

passed #199215
lava-test-4.19:qemu-armhf

00:59:19