python-netifaces: convert to the Python package infrastructure

Also, remove the "select BR2_PACKAGE_PYTHON_SETUPTOOLS", since
setuptools is a host dependency (needed to built the package), not a
target dependency.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>