Makefile: let Python tools know the out-of-tree build location via environment variable

Some of the Python tools including the test suite runner need to know when the build
location is different to the source. Intrdouce the environment variable NODE_BUILDDIR
which points to the root of the build tree. Python scripts can access this using e.g.

 os.getenv('NODE_BUILDDIR', dirname(self.config.root))

Signed-off-by: TJ <tj@khiasmos.org>
test: Make Python test suite runner aware of the out-of-tree build directory

Use the environment variable NODE_BUILDDIR to discover the correct path for the
temporary files generated by unit tests.

Signed-off-by: TJ <tj@khiasmos.org>