statemachine: Small refactoring of transition registration

statemachine: Small refactoring of transition registration

Split the guts of registerTransitions() into a registerTransition()
function. This allows a particular transition to be registered,
instead of walking the source state's whole list of transitions
every time.

Move the logic for determining whether a transition should be
registered to the state machine, since that's also where the actual
registration takes place.

Change-Id: I0496dee9454cd77b62cf2768942a82a96b320744
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>