Allow precland to work indoor

#### Issue details
I tried precland (companion board) indoor, with GPS disabled (GPS_TYPE=0, AHRS_GPS_USE=0, EK3_GPS_TYPE=3) and a lidar, but it doesn't pick up on the target. PL.TAcq shows 0, and further digging seems to indicate this is the problem: https://github.com/ArduPilot/ardupilot/blob/6a94d285de43de976e28bbab4df725c13fd96591/libraries/AC_PrecLand/AC_PrecLand.cpp#L233

https://github.com/ArduPilot/ardupilot/blob/6a94d285de43de976e28bbab4df725c13fd96591/libraries/AP_NavEKF2/AP_NavEKF2_Control.cpp#L490

It would be great to be able to fly indoor for testing, and let the camera override.

@rmackay9 @jschall

#### Version
Head

#### Platform
[ ] All
[ ] AntennaTracker
[X] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

#### Airframe type
X4

#### Hardware type
Pixhawk2.1