[new] XBee Auto-baudrate

After loosing quite some time again (this must be at least the 4th time this year) with the sys_time_usleep that is again not working (this time in all stm, last time it was lpc/rotorcraft), which breaks the XBee-Api, I changed the code to use the main periodic timer instead (at least that one will not silently get broken in some arch/firmware combination)

As I was in that file anyway, I also added automatic baudrate.

A brand new xbee out of the box (stock setting=9k6) will now work right away without any external config even with 57k6bps firmware.


PS: autoconfig for now does not work if your ground modem sends data to the aircraft...