AP_NavEKF: Increase gyro bias process noise

This is required to stop the bias estimate from becoming too static
towards the end of longer flights.
AP_NavEKF: Fix bug causing immediate clearing of diverged status on reset
AP_NavEKF: Scale divergence check with covariance

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