firmware: implement new LED scheme

[mndza]

Adds the new led_event call to allow board-defined behavior.

Merge #49 first. Fixes #39.

[mossmann]

@mndza You asked about the name led_event() and, after some thought, I think that using "status" or "indicator" might be better than "LED" for naming the parts of this that aren't directly tied to LEDs. In the future we might use other methods such as a USB vendor request to retrieve status information and/or indicate that status to the user, particularly for platforms that have fewer LEDs.