Makefile: honor PYTHON configuration properly

On some systems `python` is `python3` (for instance, Archlinux). The
`PYTHON` variable can be used to point to `python2` to have a successful
build.

The use of `PYTHON` is currently limited in the Makefile and needs to be
extended in other places:

First, pylibfdt is required to be a Python 2 binding (binman imports
pylibfdt and is only compatible Python 2), so its setup.py needs to be
called accordingly. An alternative would be to change the libfdt
setup.py shebang to python2, but the binding is actually portable. Also,
it would break on system where there is no such thing as `python2`.

Secondly, the libfdt import checks need to be done against Python 2 as
well since the Python 2 compiled modules (in this case _libdft.so) can
not be imported from Python 3.

Note on the libfdt imports: "@if ! PYTHONPATH=tools $(PYTHON) -c 'import
libfdt'; then..." is probably simpler than the currently sub-optimal
pipe.
Reviewed-by: Jonathan Gray <jsg@jsg.id.au>