The second serial port can now be used as a 4800 baud NMEA input. The new
Proto B boards have an RS-232 receiver/inverter on this input and it's wired
to CTS/RTS on the serial connector.
Also, this version improves handling of simultaneous NMEA parsing and packet
reception. Things that were OK to do in interrupt context in the
OpenTracker cause problems with the timing-sensitive demodulator routines.
Profile-switching code needs to be reorganized next, and there will probably
be more tweaks after that.