Spektrum Satelitte bind code ported from Baseflight

includes support for a hardware bind plug (PB5 pin 41)
Activate via OPTIONS="HARDWARE_BIND_PLUG" during make 
Not so easy on cleanflight. SerialRX can be configured to any port, not only USART2.

Detecting actual used port used is 'complicated' now, so best solution now is just to document this behavior.