fix(systemd): Drop old coreos-startup.target

Not much was left in here and a previous change to enable default
dependencies in issuegen was causing a dep loop. Just drop it and move
the few things left into the ebuild with everything else.
fix(README): make it closer to reality

The readme is super out of date. Move sections around to the best of my
knowledge and delete stuff that I am sure is completely different.
Merge pull request #93 from marineam/fix

unit and doc cleanup