This enables the filter to report the last known position after disarm.
The LLH location of the filters NED origin is published and should be logged every time the vehicle is armed to assist with post-flight trajectory reconstruction.
The LLH location of the filters NED origin can be set externally whilst the vehicle is disarmed.

The last known position was being output on the velocities when in constant position mode.

Also improves protection against badly conditioned variances

The two state auxiliary EKF has been replaced with a single state filter that only estimates terrain offset. The new filter fuses a optical flow line of sight rate scalar (length of the optical flow LOS rate vector) which provides a terrain offset estimate that is less affected by yaw errors.
Estimation of focal length scale factor error in flight wasn't accurate enough and will be replaced with a pre-flight intrinsic sensor calibration procedure as the scale factor error does not change over time provided the lens assembly is not adjusted.

AP_NavEKF: Remove unwanted printf

This is needed because planes arm automatically.

AP_NavEKF: Fix bug in GPS patch