python: Run flake8 at build time.

python: Run flake8 at build time.

If flake8 is installed, run it at build time.  Similar to most Makefile
targets, run it once and then only run again if the files change.

flake8 is set to ignore all error and warning types that currently occur.
Future patches will remove items from the ignore list as they are
resolved.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>