Support for GPS on Serial2 port

[kh4]

These changes add support for having a GPS receiver connected on the Serial2 port of the Teensy.

Some notes on the changes

• GPS connected on USB takes priority
• default baudrate is 9600 (I might look into autobauding later)
• GPS TX connected to pin 11 (GPIO9/RX2)
• GPS RX connected (optionally) to pin 12 (GPIO10/TX2)
• tested with BN180 GPS , in practice any NMEA GPS should work
• GPSage limit increased to 1500ms as with 1Hz update rate 1000ms can cause a glitching

[kh4]

Implements feature #3