systemd: allow mount.usr= kernel arg without mount.usrflags

Upstream commit:
https://github.com/systemd/systemd/commit/eb5800026d5a6754514fb8f8a8561b49974fc879
profiles: enable support for systemd journal upload utility
systemd: mark 217 as stable

Missed this previously, oops!
Merge pull request #991 from marineam/usr

Mark 217 stable, some minor feature updates