Uncomment the following line in setup.mk and comment out the line above
to enable the Linux build.

   export PX4_TARGET_OS	 = linux

The build uses the clang compiler by default. The final bundled executable
is mainapp located in:

   Build/linux_default.build/mainapp

When you run mainapp it will provide a list of the built-in apps. You can
type in the commands to run such as:

   hello_main start

Because the Linux build is threaded and does not support tasks or processes,
it cannot call errx, exit() _exit(), etc. It also requires unique scoped
variables to test if a thread is running or if an application should exit.
The px4::AppMgr class was added in px4_app.h for this purpose. The
hello sample app demonstrates how this is used.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

Moved the NuttX specific board files to makefiles/nuttx and added
a makfiles/linux directory with sample config and board files.

Created a makefiles/toolchain_native.mk file for building for Linux
with the native system compiler. GCC or clang can be used by setting
a flag in the file.

The Linux build creates an archive file and will build the tasks as
threads. Other code changes are required to support both task based
and thread based builds.

The NuttX source should not be required for the Linux build. The
target OS (NuttX or Linux) is selected by commenting out the desired
line in setup.mk

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

Commander: Improved preflight check routines. Running checks on all connected sensors. Re-run checks once GCS is connected.

commander gyro cal: Optimize parameter set calls and allow up to 0.0055 rad/s deviation - tuned to allow in-field calibration, but fail anyone really rotating during the step

-requiring arguments should be argc < 2

px4io firmware: Allow actuator update rates down to 45Hz, as this is exa...

This reverts commit 5fe7f766.

This reverts commit 750b02b4.

This reverts commit 3a70e7bf.

This reverts commit 9ac350a7.