AP_SpdHgtControl: added get_land_sinkrate()
AP_TECS: added get_land_sinkrate()
Plane: improve landing aim point calculation

try to account for wind and current ground speed to dynamically update
glide slope in landing to flare at the right point
Copter: remove second camera mount
GCS_MAVLINK: implement new routing rules

see description in coments
GCS_MAVLink: added routing test sketch
GCS_MAVLink: reverse sense of check_and_forward()

also increase number of routes on larger systems and improved route
learning logic
Rover: cleanup mavlink_system setup
Plane: cleanup mavlink_system setup
AP_TECS: remove lag from height demand in landing

this predicts ahead the height demand for landing, where we have a
continuous demanded descent. This removes the effect of the lag
introduced by the height demand filters
AP_NavEKF: Do not reset vertical velocity state from GPS

Doing this can cause large height and height rate errors if large GPS velocity errors cause the GPS tn be rejected for long enough to cause a timeout and reset of states.
AP_NavEKF: Use named states for velocity reset

Readibility improvement