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
Copter: remove unused GPS and baro filters

Saves 24bytes of RAM
Removing the baro filter leads to noisier Baro Alt appearing in the log
so we should make the inertial nav altitude appear in the CTUN message
Copter: rename failsafe to failsafe_radio

Makes way for separation of failsafes for throttle, gps and gcs
Copter: GPS Failsafe implemented

Switches to LAND mode 5 seconds after losing GPS if you're in a flight
mode that requires a GPS
Rover: fixed PX4 MPU6000 driver for rover
HAL_PX4: auto-create APM directory on SD card
InertialNav: remove 3sec limit on position est after losing GPS
Copter: bug fix for auto missions over 320m

get_throttle_althold_with_slew's target_alt parameter defined as int16_t
(instead of int32_t) meaning missions with altitudes >320m could wrap
around and become negative.
Copter: leonard's smoother alt-hold transition

Target altitude when entering alt-hold is based on a projection from
current alt and climb rate
GPS_UBLOX: fixed auto-config of baudrate

this fixes the setting of the baudrate on a uBlox that is configured
for 9600. It fixes the NMEA message to have a \r\n, plus sends the
NAV_SOL rate config before the NMEA baudrate config message.

This fixes issue #159
Copter: fix init of wp_verify_byte so wp alt is always required
Copter: remove unused crosstrack variables and parameters

Cross track is still calculated internally to the wpnav controller so we
should move this to a library and still report it to the ground station