Two parts to this PR.

1. Devices that are not supported by the hardware can return false from their init function and the device framework will not call their start, timeout or event functions during the main loop.
2. Event publish/subscribe
   • Events fired from the code now pass an event type so only subscribers to that event type will be notified, reducing the number of calls in the main loop.
   • Device event framework extended to allow device handlers to subscribe to certain events and their event function will only be called if that type of event is fired.