Allow POSCTL with only PX4FLOW (without GPS)

Solution for https://github.com/PX4/Firmware/issues/967

This fixes allows using FX4FLOW in POSCTL mode without GPS. Implemented EPH/EPV estimation during FLOW and GPS position estimation. Commander uses EPH to check if local position is good enough. Approach is a bit hacky, but may work. Need to review local_position / global_position topics to make it more clean.