genrandconfig: move instantiation of SystemInfo down

The SystemInfo class is instantiated globally and passed down to all
functions, but it is really only used in fixup_config. So instead,
instantiate it there.

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