chore(coreos-init): mv update-engine.service into update_engine ebuild

move the service file to the ebuild where it belongs.
feat(update_engine): restart a second after death

It is possible that update_engine will crash. Try to recover by
restarting the update_engine but wait some seconds first.
Merge pull request #49 from philips/restart-update-engine-on-failure

Restart update engine on failure