mtd: nandsim: add module param for BBT handling

I used this to check the BBT on flash together with a hack in mtdchar in
order to read bad blocks.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>