pylibfdt: Enable installation of Python module

Adjust the setup script to support installation, and call it from the
Makefile if enabled. It will be disabled if we were unable to build the
module (e.g. due to swig being missing), or the NO_PYTHON environment
variable is set.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>