instead of the derivative of the vertical position which is
not the same value when using the ekf2 estimation.

Using the position derivative resulted in delayed landing because
the value for some reason often bumped over the theshold before
slowly converging towards zero while the velocity was within expected
accuary in all these cases.

    - micro-CDR in PX4/Firmware (e0680ffe2280620b81d415328f109dc8decaa1b8): https://github.com/eProsima/micro-CDR/commit/7a76880f90dfe702f4fa1723e22d1dcb491d7b9e
    - micro-CDR current upstream: https://github.com/eProsima/micro-CDR/commit/62d95c870eafb0cccc7bf70bb8a0cbb86f125a0e
    - Changes: https://github.com/eProsima/micro-CDR/compare/7a76880f90dfe702f4fa1723e22d1dcb491d7b9e...62d95c870eafb0cccc7bf70bb8a0cbb86f125a0e

    62d95c8 2019-01-24 Luis Enrique Muñoz Martín - Refs #4438. Modified the configuration of machine endianness. (#31)

* mavlink_receiver: generate rc channel count
* mavlink_receiver: zero out ignored rc channels, add comments

Coverity scan has been offline since Monday, January 7, 2019.

 - needed if NuttX networking enabled