Move build artifacts to per-type release directories

.gitlab/ci/build_releng.sh: Move the mkarchiso build artifacts to a directory structure in the output directory, that reflects a /- setup, as discussed in #11. Add move_build_artifacts() to move mkarchiso's build artifacts to a type based directory structure. Change copy_ipxe_binaries() to copy the files to a type based directory structure and create checksums. Add set_ownership() to generalize the change in ownership, if the script is run using sudo. Change run_mkarchiso() to not implicitely copy ipxe binaries and create checksums for them, but instead call move_build_artifacts(). Call copy_ipxe_binaries() and set_ownership() globally, after run_mkarchiso() has finished.

Implements #13 (closed)