This change keeps the version-controlled 'pubkey' and 'download' files
separate from files generated via depot/create or downloaded via
depot/download. So one can remove the entire depot/ directory without
interfering with git.
Furthermore, depot keys can now be hosted in supplemental repositories
independent from Genode's main repository.
Fixes#4364
Allow the gpg utility to be overridden on the command line. For
example, './tool/depot/publish GPG="ssh 10.0.1.3 gpg" ...' would invoke
a remote gpg over SSH.
Fix#2981
