Only re-build missing incremental .mar files

Sometimes we only want to re-build a subset of incremental .mar files but that does not seem to be possible right now.

The way this could handled is like proceed with regular artifact builds: only the missing parts are (re-)built. Thus, for incremental .mar files this would mean we'd delete the ones we want to rebuilt and make incrementals-release (assuming we are in a release build process) would be smart enough to just re-build the deleted ones.