Added XBUS_MODE_B to serial rx configuration.

As the XBus support were merged into the cleanflight master, I also suggest that we add a setting for this in the configurator tab in the cleanflight-configurator tool. I added this with underscore, please let me know if this is ok or not. 
Edit: While reviewing the changes I realized that my editor (geany) had added a new-line (at the end of the file) in the source file. Please let me know if you do not want this.
Best regards
Stefan