Added Spektrum satellite binding to transmitter

Possibility to bind Spektrum satellite with transmitter without need of
separate RX.

Instructions for attaching Spektrum satellite to Naze32:
- Grey signal wire to pin 4. (pins on the left side of the Naze32)
- Black wire to ground (e.g. pin above pin 4)
- Orange wire to 3V power pin. (next to leds)

Instructions to bind the satellite:
- Make sure that satellite has been connected correctly.
- Connect Naze32 to USB
- Open baseflight and connect to Naze32 (baseflight app in Chrome)
- From Configuration tab make sure that you have selected "Enable
Serial-based receiver" and Serial receiver is correct (e.g. SPEKTRM2048
for DSMX)
- From CLI tab type: "set spektrum_sat_bind = 9" for DSMX or "set
spektrum_sat_bind = 5" from DSM2
- type "save" and after Naze32 reboot remove USB cable (=Power off the
Naze32)
- Wait a sec and reconnect the USB cable. After cold start satellite led
should start blinking and transmitter should be turned on while pressing
the bind button.
- After binding satellite led should be solid. Connect baseflight and
use receiver tab to test that satellite is working correctly.
- Final step is to go to CLI tab and type "set spektrum_sat_bind = 0"
and then type "save". This must be done so that satellite doesn't go to
bind mode when Naze32 is repowered.