system: initialize suspend backend when suspend is called the first time

The nyx system module is also used for other things like partition erasing and we don't
want the suspend backend to be setup just because the user wants to do something
differently without caring about using the suspend functionality.

Signed-off-by: Simon Busch <morphis@gravedo.de>
system: implement new asynchronous suspend API

Signed-off-by: Simon Busch <morphis@gravedo.de>