This is a step towards isolating time from the system.

    - v2.0 in PX4/Firmware (3256e67fdc5e86b5d8c6378ea24ff8a065b65ba8): https://github.com/mavlink/c_library_v2/commit/c38176a0a63556815ea117c6877ce2dd18739231
    - v2.0 current upstream: https://github.com/mavlink/c_library_v2/commit/175ab6725cc0d96827d04ffeeaf3a7797a9abdbf
    - Changes: https://github.com/mavlink/c_library_v2/compare/c38176a0a63556815ea117c6877ce2dd18739231...175ab6725cc0d96827d04ffeeaf3a7797a9abdbf

    175ab67 2018-12-17 PX4BuildBot - autogenerated headers for rev https://github.com/mavlink/mavlink/tree/eaa914c7d9789bc37c9a6a59a5bd1171634bd429
4d5fa0d 2018-12-14 PX4BuildBot - autogenerated headers for rev https://github.com/mavlink/mavlink/tree/48df7ea08abb5d87df6ccb1f9a69b91ff3b89c7a
b65cfcd 2018-12-14 PX4BuildBot - autogenerated headers for rev https://github.com/mavlink/mavlink/tree/b671d3c24d5fc6aa964cd07a4a46566d4dbd7862
8473fe2 2018-12-13 PX4BuildBot - autogenerated headers for rev https://github.com/mavlink/mavlink/tree/2b9d470011a0d62fbfc3183ee172201e5170f7af
5a186d4 2018-12-13 PX4BuildBot - autogenerated headers for rev https://github.com/mavlink/mavlink/tree/68ed09da2e341b2932d9c9d761d7baa811191dc2

    - matrix in PX4/Firmware (3e5d66580520e5ae64555c5fc9252c6c661389e3): https://github.com/PX4/Matrix/commit/9c0acfba36a928b7b002eea3a9dca54a5a0c1250
    - matrix current upstream: https://github.com/PX4/Matrix/commit/18fba8221cc1840964c43f04d8c2aa21857e1b9b
    - Changes: https://github.com/PX4/Matrix/compare/9c0acfba36a928b7b002eea3a9dca54a5a0c1250...18fba8221cc1840964c43f04d8c2aa21857e1b9b

    18fba82 2018-12-16 Matthias Grob - quaternion: improve comments
a4f39c0 2018-12-16 Matthias Grob - quaternion: reuse existing dot product

* the bearing difference between the waypoints and aircraft to landing point should be wrap_pi'ed

* mavlink_messages: remove LOCAL_POSITION_NED_COV stream

* mavlink_messages.cpp: add ODOMETRY stream

* add MAV_ODOM_LP parameter to activate odometry loopback

* EKF2: add vehicle_odometry publisher

* Replace VISION_POSITION_ESTIMATE loopback with ODOMETRY

* LPE: add vehicle_odometry publisher

* set vehicle_odometry local_frame field

* mavlink_messages.cpp: ODOMETRY frame_id depends on MAV_ODOM_LP

These files are copied by accident and then discovered by shellcheck.

Shellcheck complained about returning -1 because that's essentially 255.

This was required for shellsheck.

The startup file rc.board is copied to the build/../genromfs folder
after the pruning of the files happened. Thus, about 2000 bytes of
comments and whitespace were included in the ROMFS.

This commit moves pruning of the files in ROMFS to a separate cmake
custom command which happens after the ROMFS files have been copied and
the extra files have been added.

Testing showed that this change only affected the rc.board file as
expected.

It is only actually sent out when the orbit is running.

tone_alarm was started before the parameters were loaded, and the first
tune was played before that as well. CBRK_BUZZER was then read as default,
ignoring the user-configured value.

We now start tone_alarm after we load the parameters. Note that a
previously published startup tone or SD card error will still be played.

This is more accurate, because if we use -1000 as lower bound, a very large
yaw command can reduce thrust more than needed.

The difference can be seen in the following example
(with roll/pitch airmode):
p_dot_sp = -1.0 # roll acceleration
q_dot_sp = 0.9 # pitch acceleration
r_dot_sp = -0.9 # yaw acceleration

makes it easier to compare.

Yaw airmode w/o arming switch is most likely not what you want and will
lead to surprising results.

To run:
cd src/lib/mixer
make tests

This will validate the C++ implementation by taking the python
implementation as ground-truth. It runs through various actuator control
command values for all airmode variations and several mixer types.

The python script also allows to prototype new mixer algorithms.

It is not integrated into the existing build system, because it's easier
to use that way, with less dependencies, and faster testing workflow.
It could however be a bit more integrated.

Reference: https://github.com/Auterion/Flight_Control_Prototyping_Scripts/tree/master/control_allocation

- avoid including <px4_defines.h> from the math headers
- avoid driver include <drivers/drv_mixer.h> from the mixer