Improve maintainer scripts of firmware-nonfree

1. add option --show_licence to debian/bin/check_upstream.py: in case a file was classified as undistributable, this displays parsed licence data (which may be wrong due to bugs in the WHENCE parser of the linux-support deb, to be fixed separetely)
2. debian/bin/gencontrol.py: replace bare python exceptions by meaningful error messages
3. improve help/documentation of debian/bin/release-update
4. automatize repacking of orig tarball (repack without non-redistributible files)
5. improve Build-Deps with items gathered from debian/salsa-ci.yml
6. implement and document 'ignore-files' debian/config/*/defines option main target: notes or separate license files, which we already distribute in our packages.