genrandconfig: replace kwargs with explicit arguments

kwargs is a left-over from the use of docopt, it's better to use
argparse's Namespace object directly.

In addition, most functions use just one or two fields of args, so
these can just as well be passed directly as arguments to the function.
Particularly for outputdir it doesn't make sense to reconstruct it all
the time.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>