Refactoring for multiplatform support

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>