NAND: Scan bad blocks lazily.

Rather than scanning on boot, scan upon the first attempt to check the
badness of a block.  This speeds up boot when not using NAND, and reduces
the likelihood of needing to reflash via JTAG if NAND becomes
nonfunctional.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>