subprocess.communicate returns bytes instead of a str which is not the
same for Python3. Therefore, we need to decode the bytes.

The folder src/firmware was not intuitive. Why would the binaries for
SITL be inside a src and why even inside a src/firmware folder. Also,
the rootfs was put there which made it even more confusing.

The CMakeLists.txt files are moved into cmake/ and get now called from
the main CMakeLists.txt.

Instead of just sending commands, qshell will now also wait until
the command has finished on QURT and sent back a return value. This will
allow all modules on the DSP side to be spawned from the Linux side
meaning that we only need one config/startup file instead of two.

Previously the script failed if the folder on the destination was not
already existing. This therefore makes pushing easier.

This adds the possibility to spawn PX4 modules out of bash. Basically,
the main executable can now be started as a server/daemon or as a
client.
The server replaces the existing functionality of the main exe with
the pxh shell, however, it also opens a pipe that clients can talk to.

Clients can run or spawn PX4 modules or commands by connecting to the
server over the pipe. They clients will get the stdout and return value
of their commands via a client specific pipe back.

This work will allow to start all modules using a bash script similar to
the way it is done in NuttX where the NuttShell scripts the startup
scripts and starts the modules.

This is a first step to use the new shell capabilities for SITL.
The new startup bash script rcS merges (and therefore replaces) the two
existing scripts rcS_gazebo_iris and rcS_jmavsim_iris.

More cleanup will be necessary for the rest of the SITL startup scripts.

Instead of different mainapp.config and px4.config files, we can now use
a unified rcS bash script which starts all the modules based on
parameters, mainly the SYS_AUTOSTART param.

The comment was wrong that argv needs an additional 0 termination.
Instead it needs a nullptr at the end.

The px4_task_spawn_cmd function got a cleanup while debugging, however,
no functional changes.

These drivers are supported by the community, so they go into the 4100
config.

… script aliases)

This one was probably wrong and untested. Installing needs revisiting.

Instead of using non-working install commands, the mixer and startup
files are now copied as part of the build in cmake.

chroot is removed because it hasn't been used anywhere and seems
untested.

The client pipe is supposed to be specific (by UUID), so the path
shouldn't exist already.

This is a protection to avoid filling the memory if we are entering a
lot of commands (e.g. auto-generated).

Sometimes the client ends up reading more than one packet in one read.
The parsing is not made for this and would require a (ring)buffer for
it.

The solution of this commit just reads as much as needed from the pipe
which avoids having to do buffering and parsing.

This changes the paths in sitl_run.sh quite a bit to allow the px4
binary to run in the rootfs directory which should make it convenient
and very close to the NuttX variant.

Also main.cpp got a big cleanup after the big rebase with some
conflicts. Quite some functionality was removed but it has yet to be
seen if it needs to be re-added.

Before DEBUG and INFO log levels where inverted which didn't make much
sense in my eyes.

- Support for different models using the unified startup
script rcS.
- Support to choose the estimator by setting the environment variable
  PX4_ESTIMATOR.
- Support to choose the logger by setting the environment variable
  PX4_LOGGER.

Instead of having all of the C++ code inside the Python file it is
nicer to have a separate template file with the C++ headers, etc.

This allows to do custom formatting but is still transported over
sockets to clients.

Hopefully this makes it easier to read and change in the future.

This makes the unit test compile and link again after the bash changes.

This allows to use a custom startup file for testing.

The shutdown command needs to be a proper systemcmd, otherwise the alias
and symlink generation doesn't work and we end up calling shutdown of
the host computer which is to be avoided.

There is no good reason to use a specific test mixer, except more cmake
code around it. Therefore just use the same mixer as default, and at
some point px4fmu_test and px4fmu_default can get merged

This cleans up the logic behind the symlinking and creating directories.

This was fine with cmake 3.6 but did not work with cmake 3.2.2

cmake -E copy does not support copying multiple files with versions <
3.5. Therefore, just use cp for now.