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
I guess the 0x8000 vs (1<<15) is a matter of taste. ;) Most of the time I have used HEX-numbers as masks and I did not really think too much of it. 
About the CRC method, I just pulled it from the specification found here: 
http://www.multiplex-rc.de/service/downloads/schnittstellenbeschreibungen.html
It might (propaly) be some kind of standard, but as that was not known by me I included it into the xbus-code instead of lifting it out to a more general location.