This provides consistent behaviour for a range of gyro bias process
noise settings and automatically adjusts sensitivity as filter learns bias

This is required to stop the bias estimate from becoming too static
towards the end of longer flights.

At the moment, the copter sonar set up does not have access to the
raw sonar readings so this code sets the voltage field to zero.

this provides a common way of dealing with UART setup for a GCS
instance. It includes code to cope with SiK radios stuck in bootloader
mode.

this prevents an initialisation error, and is reasonable in flight too

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>

we need to check health of each accelerometer separately

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>

will be used to indicate AHRS health

Divergence is now detected by looking for very large changes in the gyro
bias. This will cause the filter to be reset and declared unhealthy for
10 seconds.

Don't reset filterDiverged status immediately during reset

Set filterDiverged true if covariance blows up

Add fault status reporting

this will be used to report when the AHRS subsystem becomes unhealthy

logs all accel data at high rate to SD card

This makes it consistent with rover and copter and clarifies where the
pin is on the Pixhawk

Also prepended APM2 to existing values.  Added A2 to list to be
consistent with plane and copter

Also prepend APM2 to existing definitions

This is now part of the AP_Relay library

This resolves an issue in which the mission would not automatically
start from the beginning if it had previously been run to completion

Thanks to EmileC for spotting this