.gitlab-ci.yml: use "extends" keyword

Replace all YAML anchors with the new "extends" keyword because it is
more readable and more flexible (it works across configuration files
combined with the new "include" keyword).

Readability is more meaningful in .gitlab-ci.yml.in.
In the part of .gitlab-ci.yml that is auto-generated by 'make
.gitlab-ci.yml' keep the keyword in the same line of the job name.
So instead of this:
 zynqmp_zcu106_defconfig:
     extends: .defconfig
 tests.boot.test_atf.TestATFAllwinner:
     extends: .runtime_test
Use this:
 zynqmp_zcu106_defconfig: { extends: .defconfig }
 tests.boot.test_atf.TestATFAllwinner: { extends: .runtime_test }
Do this to to keep .gitlab-ci.yml easier to be post-processed by a
script.

Signed-off-by: Ricardo Martincoski <[email protected]>
Cc: Arnout Vandecappelle <[email protected]>
Cc: Thomas Petazzoni <[email protected]>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <[email protected]>