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 see what you mean. The only speed optimization would then be to not have to calculate "crc" (variable in my code) and compare to a constant (i.e. zero) instead of (crc == inCrc). This would however include two more bytes in the loop of crc-calculations (including the small crc-loop in xbusCRC16), hence I do not really see the optimization benefit as the result (i.e. checking for bad messages) will be the same.