gitlab-ci: do not run real build on master branch
In our GitLab workflow we don't use merge commits, but rebasing only. A merge action does not create a new commit object accordingly. Thus, the pipeline runs on exactly the same commit again after merging, and even produces an identical build artifact.
We don't want to waste our valuable build time with redundant builds, so with this commit we eliminate the build on the master branch.
For the build result of the commit to still be visible on the master branch, we introduce a new job "build:merge-request" which mirrors the result from the merge-request to the master.
Change-Id: Iabd9b6862b09e7fcbb114b5d25e38f76292a9709