AP_NavEKF3: EKF ignores rangefinder for terrain estimates if RNGFND1/2 are not configured downward facing

**Issue details**
EKF3 (and probably 2 also) currently ignores rangefinder readings if one of RNGFND1 or 2 _ORIENT is not set to 25 (Downward facing). This means if, for example, RNGFND4_ORIENT = 25 is the first downward-facing rangefinder, the user will not be able to switch to a loiter in an optical-flow-based setup.

This is easily reproducible in SITL. The culprit code is here: https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp#L37

**Platform**
[ x ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine