pylibfdt: Use Makefile constructs to implement NO_PYTHON

The current mechanism uses a shell construct, but it seems better to use
a Makefile approach.

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