PX4IO: Added support for JR XBus Mode B

[omcaree]

JR XBus Mode B uses the same serial port parameters as the DSM port (3.3V logic, 115200, 8n1), this update adds support for decoding it.

XBus Mode B is effectively the SRXL protocol, using the Multiplex 12-channel format (start charachter 0xA1)

Decoding of XBus packets only occurs if a valid checksum is received, so this should not interfere with DSM packets.

[doc2005]

it would be great if that were merged, would be a great improvement!

[rickpayne]

I would appreciate this one being merged too.

[cromartic]

It would be great if that were merged.

[pedrodparkes]

hey, can somebody say exactly, was it implemented or not? Can I somehow make pixhawk work with JR X-BUS?