EKF2: added absolute range altimeter param

[tuloski]

Added support for absolute distance altimeter or for range finders mounted on a gimbal such that they are always vertical. No need to rotate from body to earth frame.
Used in case of absolute distance provided, no matter of which orientation the copter have.
Be careful that the max limit in roll/pitch to have a valid measure is still active.
A parameter (EKF2_RNG_ABS) is used to enable this feature.

WARNING: requires also the pull request in PX4/ecl with the appropriate modifications to EKF lib.

WARNING2: I was able to see the param in QGC, but I wasn't able to see the meta. Probably it's a QGC cache problem. Double check that metadata is correct.

[tuloski]

This is the PR in PX4/ecl: PX4/PX4-ECL#547.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[julianoes]

Thanks for the pull request.
This is probably helpful, have a look @RomanBapst

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Thank you for your contributions.

[LorenzMeier]

Thanks for your earlier contribution. Unfortunately as the project has overall evolved quite a bit, this change doesn't apply any more. Closing stale pull requests like this one is part of us working aggressively to bring down the PR review time so that we will be able to merge or reject PRs in the future in a much more timely fashion.