Support for simultaneous GPS and Spektrum satellite

Added support for simultaneuous GPS and Spektrum satellite (or
SBUS/SUMD):
- Features needed to enable: "Enable Serial-based receiver", "Enable 3rd
serial port" and Enable GPS
- Naze32 pins: Spektrum satellite RX pin 4. GPS TX pin 5 and GPS RX pin
6.
- GPS is using soft serial so it's maximum baud rate is 19200 (current
softserial implementation). Default baud rate is 9600, but it can be
changed from CLI with command set softserial_baudrate=19200. After setup
I recommend to power off for some time so that GPS will work.

Added more channels to Spektrum satellite:
- DSMX satellites (SPEKTRUM2048) support up to 8 channels.
- DSM2 satellites (SPEKTRUM1024) support up to 7 channels.