Introduce a makefile lint rule

This runs pylint over all the python files. Yes, it takes a long while,
but it's the only way to properly analyze the source codes as only in
this way pylint can see all uses of the various modules/classes/etc.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>