[OE-core] psplash activation state w/ systemd

[Alex Kiernan]

I've a branch with systemd 245 on it which fails testing because
psplash gets restarted all the time.

But ignoring the systemd 245 piece, it looks to me like psplash could
be restarted under systemd 244 too as the main process exits and our
units aren't marked RemainAfterExit. I haven't figured out what's
triggering it in 245 and not 244, but I suspect it's something similar
to this:

https://github.com/systemd/systemd/commit/9fd32ff7d363945fbf8fdae0128702b995127558

Given where we are in the release cycle and how painful psplash has
obviously been, I'm inclined to leave it until after dunfell ships and
then do it as part of the whole 245 upgrade?

Equally if you see psplash flapping during tests, it's almost
certainly this problem.

-- 
Alex Kiernan