Fixing current meter, vbatCycleTime was not being reset and the ADC
index was wrong.

See 5cc9750d123c786ededfb9e69b3f8e2033181b81
Latest binary with fixed current sensor.
Split navigation functionality from io/gps.c into flight/navigation.c.

gps.c now only has code that deals with gps hardware, state and
messaging.
navigation.c  now only has code dealing with flight
navigation/waypoints/home/hold/etc
CC3D - Support USART3 instead of USART2.

This has highlighted that the existing codebase is quite targeted
towards systems that use USART1 and 2.  Annoyingly the inverter is on
USART1 and the sbus code requires callbacks so still won't work yet.
Update documentation.
Adding CC3D and Naze32 preliminary docs.
Adding buzzer documentation.
Update serial port handling for boards with only 2 ports.
Replace qsort with simple bubble sorting function to reduce code size.