DataFlash: report log number when starting a new log

helps with test sketch
DataFlash: fixed test sketch build and run
Rover: implement RSSI_PIN

route RSSI value from receiver to MAVLink
AP_HAL: update AnalogIn example to show voltages not raw values
Rover: remove scaling of voltage pin
autotest: added skid steering support to rover simulator

Very rough initial implementation. Needs to be tweaked for more
realistic behaviour later.

thanks to Greg Brill for pointers on this.
autotest: added skid steering version of rover sim start script
Rover: initial implementation of skid steering for APMrover2

this adds SKID_STEER_IN and SKID_STEER_OUT parameters for controlling
skid skeering control and output
autotest: improved skid steering test scripts