Add CI integration to build profiles

Add a build stage to the gitlab-ci for generic merge requests and for builds based on changes on master, scheduled builds and pushes of tags.

The main script in use is a slight modification of arch-boxes' build-host.sh, for which code-sharing should be setup (else one could also think of an even more generic, packaged solution for this). The custom script to build both the baseline and releng profile in parallel also creates various checksums for the artifacts.

Testing of the created artifact (probably using run_archiso) will take place in a separate merge request (tracked in #94).

Closes #2 (closed)