feat(systemd): don't do STATE stuff when /usr is a mountpoint

This is to make it easier to share code right now, just skip all of this
STATE work if /usr is a mountpoint.
feat(Makefile): introduce install-usr

The /usr move means that a few things are going to have to look
different. Namely tmpfiles and some configs like ssh. Make a different
target for the ebuild to use.
Merge pull request #34 from philips/usr-init

Changes for /usr