Explore on integrating more with systemd

[say-paul]

thread: https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/message/KFWF5FX2PXIYFTOFEYLC5ORWUWUQHUOX/

There is a significant overlap between the systemd framework for this,
i.e. boot-complete.target, and the greenboot framework. Greenboot
partially ties into the systemd framework, at least by ordering
before boot-complete.target, but it also partially sidesteps it, by
managing the reboot itself. Since you're investing new work into
this, would it be possible to remove this duplication? It'd definitely
be beneficial for users to have a single documented framework for
this, with a uniform interface on all systems. Also, obviously with
less duplicated work, we can have one system that works better.