Use Bluetooth LED to show when BT connected or not

[mozzwald]

Request by phigan on AA:

It would be really nice if there was some kind of indication of connection with the other end when in SIO2BT mode. Perhaps when you first hold down A to get into the mode, it can display the blue LED solid for a couple seconds, but then after that it would flash if disconnected and stay solid only if connected.

This will require a rewrite of the LED code to have it's own task so as to not delay any other critical functions while servicing the blinks.