feat(systemd): add local.target and friends

/media/state/units is a place for people to put unit files that will be
automatically enabled and started if they are WantedBy local.target
fix(coreos-startup): this should be part of basic.target

basic.target is for "Usually this should pull-in all mount points, swap
devices, sockets, timers, and path units and other basic initialization
necessary for general purpose daemons"

Seems like a better fit.
fix(coreos-startup): remove update_engine

update_engine can take care of itself. Let it start with
multi-user.target
fix(coreos-startup): remove all daemons

coreos-startup only deals with mountpoints and filesystems now.