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
BTW: is it really neccesary to specify `mode B` here?

After quick look at documentation there is one type of XBUS framing (type/version byte + payload + crc) so xbus.c may be modified to handle all types of XBUS in future (and no user configuration is necessary), so just SERIALRX_XBUS may be enough?