.flake8: add config file for Python code style

.flake8: add config file for Python code style

We recommend wrapping at 80 columns but we accept 132 columns when it
makes more readable.

Follow up of discussion at this thread:
http://lists.busybox.net/pipermail/buildroot/2017-October/203624.html

Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>