platform/avr-ravenusb: Move management of LED status to dedicated thread.

This change creates a new thread for managing the LEDs on the Atmel
RZUSBStick hardware. It provides a simple interface for manipulating
the LEDs and makes sure that they are managed in a visually-pleasing
manner, while also providing a hardware abstraction layer.

I've been holding onto this one for a while. Once this is merged in, I think it
will be time for some of the more significant jackdaw patches.