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

The code had g.rc_3.control_in which would bypass the channel mapping and meant
arming could only occur if the throttle was always mapped to channel 3.
Updated to use the channel mapping object.

Guided-Spline has been replaced by Guided-PosVel

Also set auto-yaw-mode to yaw-hold only when posvel controller is
started instead of on every iteration.
added GUIDED_POSVEL_TIMEOUT_MS definition.
removed deprecated trigger_xy call.
Also use wp_nav.get_speed_xy in place of get_speed_param

no functional change