In commit
eb67686b
the TPA scaled integral gain was reverted back to the
normal unscaled gain which rendered the I part of TPA
useless.

* Lists ROMFS files explicity instead of using GLOB_RECURSE

Previously, when ROMFS files that were not airframes were touched, the ROMFS
would not be rebuilt. The ROMFS files are now specified explicityl in a
CMakeLists.txt file that is located in the root ROMFS directory.

Now when one of the ROMFS files is touched the whole ROMFS is rebuilt.

When new files are added to the ROMFS, they need to be explicity added to
the CMakeLists in the ROMFS root directory.

* ROMFS: adds individual CMakeLists files in each subdirectory

Also moves the temporary ROMFS build directory to ${PX4_BINARY_DIR}/ROMFS/genromfs
so that the cmake_install.cmake files and the CMakeFiles directories (generated whenever
are not add_subdirectory() is called) are not generated in the temporary ROMFS directory
from which the ROMFS binary is created.

* cmake ROMFS generate add px4_add_romfs_files function

* ROMFS CMakeLists: adds explanatory comment to px4_add_romfs_files function

* ROMFS CMakeLists: updates copyright headers

Since the tune library also contains logic how tunes can be overriden,
a user of the tunes library cannot know the strength of the current tune
without replicating some logic. Easy solution is to add strength to the
output of Tunes::get_next_tune().

Otherwise the only way of interrupting a repeated tune would be with the override flag, which should only be used rarely.
Now repeating tunes have lowest priority, followed by one-shot tunes, followed by anything with an override flag.

Override flag must still be set to true!

Bugfix: This fixes a bug where libtunes might indicate that there are more tones to play even after the last note of a tune.

Custom msg tune can override any tune playing, regardless of the tune_override flag.

Revert the disabled d term filter which needs to be enabled
after reducing IMU filtering and was probably unintentionally
during a rebase.

 - fixes coverity CID 196758

 - fixes coverity CID 260383

 - fixes coverity CID 264261

 - fixes coverity CID 264263

 - fixes coverity CID 264259

Signed-off-by: Roman <bapstroman@gmail.com>

If not in air-mode the mixer is not able to apply positive boosting
and roll_pitch_scale is recomputed to apply symmetric - reduced - thrust.
This has the consequence to cut completely the outputs when the thrust is
set to zero.

this avoids calling param_get() on every loop iteration.