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.
avr-ravenusb/status-leds: Code style updates.