[RFE] Use common boot complete systemd target

[pothos]

Current situation

update-engine.service marks the booted partition as working after ~2 mins

Impact

Users would have to add a unit dependency for update engine to add a required service that must come up for the booted partition to be marked as working. However, this is Flatcar specific.

Ideal future situation

We use the systemd boot complete target so that users can add dependencies on the same common systemd target instead of a Flatcar-specific target.

Implementation options

Make update engine part of this target but consider to move the timer out of update engine and use a shared generic implementation.

Additional information