pylibfdt: Use Python 2 in Makefile

pylibfdt needs Python 2 to build.
Replace $(PYTHON) with $(PYTHON2) in pylibfdt Makefile
to ensure Python 2 is used to build it.

This fixes build on systems where Python 3 is the default version
of the "python" interpreter.
Reviewed-by: Simon Glass <sjg@chromium.org>