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>
16 jobs for next in 167 minutes and 53 seconds (queued for 36 seconds)
Child pipeline (parent)
Status Job ID Name Coverage
  Build
passed #199208
build-4.19:board-beagle-bone-black

01:16:46

passed #199206
build-4.19:board-hikey

01:33:17

passed #199200
build-4.19:board-qemu-amd64

01:16:40

passed #199204
build-4.19:board-qemu-arm64

01:20:30

passed #199202
build-4.19:board-qemu-armhf

01:31:03

passed #199209
build-4.19:board-x86-64-efi

01:17:59

passed #199211
build-5.4:board-qemu-amd64

01:02:13

passed #199212
build-5.4:board-x86-64-efi

00:56:18

 
  Test
passed #199221
lava-test-4.19:board-beagle-bone-black

00:14:53

passed #199219
lava-test-4.19:board-hikey

00:10:41

passed #199223
lava-test-4.19:board-x86-64-efi

00:15:31

passed #199213
lava-test-4.19:qemu-amd64

00:29:16

passed #199218
lava-test-4.19:qemu-arm64

00:44:48

passed #199214
lava-test-4.19:qemu-armhf

01:16:28

passed #199226
lava-test-5.4:board-x86-64-efi

00:24:19

passed #199225
lava-test-5.4:qemu-amd64

00:24:55