use baro for the ekf alignment if range finder is primary height source

[ChristophTobler]

As discussed in #290, I changed it to use baro if range finder is primary height.

[priseborough]

Do you have a test log?

[ChristophTobler]

@priseborough
Here's a log where alignment failed (old):
http://logs.px4.io/plot_app?log=56189966-f135-4be3-b187-4c2a4dcb1464
And here's one with this PR:
http://logs.px4.io/plot_app?log=7271d90a-642f-4e95-aceb-244bfb69733c

[priseborough]

I have verified that it is able to switch to range finder when it is available.

[jlecoeur]

@ChristophTobler @priseborough @mhkabir I think this introduced another bug in height estimation.
Was this tested for flight at fixed altitude above ground with baro+sonar and no GPS?
In my scenario I give an altitude command of -1m in offboard mode.
After this PR, I have a large offset in the actual drone altitude (it either descends to the ground, or flies too high at approx 2m)
By checking the commit before this PR, the 1m height is correctly tracked (but of course #290 is back!)

[ChristophTobler]

@jlecoeur Thanks for reporting. Can you share the two logs?

[jlecoeur]

Not at the moment. I can make new tests tomorrow.

[ChristophTobler]

I think I know what the problem is. Before, https://github.com/PX4/ecl/blob/ec2b54fde7a590178483ab9623459109003a4a44/EKF/control.cpp#L759-L765 did not run because _control_status_prev.flags.rng_hgt was already true because of https://github.com/PX4/ecl/pull/292/files#diff-0871d0235139c63531138e2793b76de7L213. Hence, the _hgt_sensor_offset was zero.
However, with the new changes, https://github.com/PX4/ecl/blob/ec2b54fde7a590178483ab9623459109003a4a44/EKF/control.cpp#L759-L765 runs after the initialization with baro. Therefore, _hgt_sensor_offset is non-zero.

This is not a problem with a good rangefinder as it will output the minimum distance when close to ground (or at least negligible). I suspect that your sonar will output a lot of noise when on ground and therefore _hgt_sensor_offset is initialized with a wrong value.