Serial auto-enable works on B3

[Mitch Bradley]

This is just a success report on the new serial auto-enable feature on 
B3.  I removed the mistakenly-stuffed power resistor (pr148) on the 
serial_en signal and modified a USB serial dongle (cut the trace that 
goes from 3.3V to pin 1 of the header, soldered a jumper from pin 1 to 
pin 4).

With the dongle connected to the serial connector on the B3, it comes up 
with serial enabled.  Disconnected, serial is disabled.

I haven't tried the pencil-enable technique.