pylibfdt: Use environment to pass C flags and files

At present setup.py adjusts its command line when running, so that the
C flags and file list can be passed as arguments. Pass them in environment
variables instead, so we can avoid this messiness. It also allows us to
support the 'install' command.

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