This prefers gitlab CI's target branch from the env, and falls back to
the old target branch detection mechanism (using pmaports.cfg, etc) if
it's not set. This fixes CI in gitlab where the target branch is
something different than expected, e.g. if using stacked branches.
[ci:skip-build]: already built successfully in CI
In 11682fb0ab, the timeout was set to 10
hours. This means that the CI no longer just runs for one hour. However,
we are still limited by GitLab's runners which only give us three hours
of CI time per job, so given the status quo, this is the most likely
duration for someone who hasn't set up their own CI runners.
Adjust all CI scripts that make use of .ci/common.py to
'pmbootstrap ci'. Move all scripts that are not direct entry points to
running CI scripts to .ci/lib.
Comment out the dtb check, as it is failing. Apparently it didn't run
properly before. Let's fix this after this CI change is done.
