fdt: Move to setuptools

The distutils package is deprecated. The upstream libfdt repo uses
setuptools for building the pylibfdt module, so bring in that code,
suitably modified for U-Boot. Also bring in the README.

The modifications include setting the version correctly, making use of
the environment variables provided by the Makefile and various tweaks
to the directories.

Note that the version omits the minus character at the start of
EXTRAVERSION, since this creates a warning. The build is really just used
within U-Boot itself, so it doesn't matter too much if the version matches
upstream, or exactly matches U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>