Initial Linux support including execution shell

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>