Added initial version of JRPropo XBUS Mode B support.

Hi there!
I am using JRPropo (http://www.jrpropo.co.jp/english/propo/XG14/) transmitter for some of my models. I have been waiting for XBUS (mode B) support for cleanflight (and other firmwares) but I finally came to the conclusion that I would have to add it myself...so here it is. Initial tests working nicely on my system with 12 channels. Would be glad if more people could help with testing.

Tested with these cli-settings (on a NAZE32 rev 4) using a serial cable from rx to flightcontroller:
set serialrx_provider=5
set serial_port_2_scenario=3
feature RX_SERIAL

NOTE: This is actually multiple protocols that uses this standard. Known under the UDI, Jeti, Multiplex and SRXL protocols. Hence with more help/test multiple new brands of tx/rx systems can be supported. I know there are some small differences between these dialects of the protocol (sync byte/id etc) and hence I named it "xbus" as this is the one I have tested. 
More info can be found here: http://www.rcgroups.com/forums/showthread.php?t=2018101

Best regards
Stefan Grufman
According to http://www.multiplex-rc.de/service/downloads/schnittstellenbeschreibungen.html vendor/protocol part is first nibble (0xa0), verion is lover nibble (0x01). Multiplex uses 0xA1 for 12 channel, 0xA2 for 16 channel, everything other is exactly the same. It would be great to write the driver so that 0xa2 addition is simple in future ..