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 
I feelt like this alredy. For the STM32F103 we only have 2 hardware Serial ports. It does not make sense to use a softserial with the actual baudrate limitations for the serial RX. Should we add someting in the DOC directory or document this limitation in the code?