In mkosi, we've been having CI failures caused by
systemd-machine-id-commit.service timing out. Let's bump the timeout
for it and systemd-rfkill.service to 90s which we also use for other
oneshot services to avoid transient failures on slower systems.
Add a new target for synchronizing units that wish to run once during
the first boot of the system. The machine-id will be committed to disk
only after the target has been reached, thus ensuring that all units
ordered before it had a chance to complete.
This makes things a bit simpler and the build a bit faster, because we don't
have to rewrite files to do the trivial substitution. @rootbindir@ is always in
our internal $PATH that we use for non-absolute paths, so there should be no
functional change.
