- cleanup airspeed validity check logic

Fixed ICM20948 magnetometer only option.
Specified bus for internal mpu9250 on Pixhawk 2.1 to avoid premature detection of
other device.

- I enabled it by default mostly for testing purposes
- Disabling it reduces resource usage as most setups don't need it

Fixed an issue with a local variable that should have been a member variable. Increased the P term to reduce rise time. Removed feedforward as it was not neccessary. Removed some custom commands that didn't serve much of a purpose.

* increase pmw3901 publish rate and max_ground_distance
* set work queue to high priority

PositionControl: update comment about the order of thrust and position

mc_pos_control: reset setpoints to NAN if required

MulticopterLandDetector: consider to be landed if vehicle is not armed

mc_pos_control: initialize landing struct with landed

mc-pos-ctrl: adjust thrust-setpoint and yaw during ground-contact/maybe-landed without capturing
that information within vehicle-local-position-setpoint topic because that information
does not belong to user intention

PositionControl: set local position/velocity setpoint to NAN if not used in the control pipeline

mc-pos-ctrl: Fill vehicle_local_position_setpoint_s structure as follow:
The message contains setpoints where each type of setpoint is either the input to the PositionController
or was generated by the PositionController and therefore corresponds to the PositioControl internal states (states that were generated by P-PID).
Example:
If the desired setpoint is position-setpoint, _local_pos_sp will contain
position-, velocity- and thrust-setpoint where the velocity- and thrust-setpoint were generated by the PositionControlller.
If the desired setpoint has a velocity-setpoint only, then _local_pos_sp will contain valid velocity- and thrust-setpoint, but the position-setpoint
will remain NAN. Given that the PositionController cannot generate a position-setpoint, this type of setpoint is always equal to the input to the
PositionController.

mc_pos_control: switch to designated initializer for landed

It's less error prone because it produces an error on every discrepancy.

Fix math::constrain usage

Fix second constraint

Since SBUS does not have CRC, we can use timing information to improve
parsing reliability and reject unexpected bytes.

 - <cfloat> isn't available in the NuttX c++ library

 - <cinttypes> isn't available in the NuttX c++ library

 - fixes #11125

fix home position timestamp to be the timestamp that the home was set and not when the message was sent

zeroed uninitialized fields on home_position, mavlink_rc_channels_override, mavlink_mission_item, mavlink_mission_item_int

It turns out that `sendto` does not work for TCP on Cygwin-Windows,
instead we need to use `send`. This required some refactoring since we
need to have the internet protocol and the port stored as a member
variable.

This should fix that lockstep SITL simulation was not working on
Windows.

    - micro-CDR in PX4/Firmware (b043cfac5e8635d9017ad6a13f45795ad6fda6fb): https://github.com/eProsima/micro-CDR/commit/8b3546e8848e4c07c16ec55730d3ad1aa94396ab
    - micro-CDR current upstream: https://github.com/eProsima/micro-CDR/commit/7a76880f90dfe702f4fa1723e22d1dcb491d7b9e
    - Changes: https://github.com/eProsima/micro-CDR/compare/8b3546e8848e4c07c16ec55730d3ad1aa94396ab...7a76880f90dfe702f4fa1723e22d1dcb491d7b9e

    7a76880 2018-12-11 Julián Bermúdez Ortega - Badges (#24)

- the flash-backend is selected by specifying nullptr as file or -1 as fd
- the default file is nullptr, and thus the FLASH
- 'param select' has no effect, FLASH is always the default

Thus there should be no functional change to existing setups.