Remove legacy NAND and disk on chip code.

Legacy NAND had been scheduled for removal.  Any boards that use this
were already not building in the previous release due to an #error.

The disk on chip code in common/cmd_doc.c relies on legacy NAND,
and it has also been removed.  There is newer disk on chip code
in drivers/mtd/nand; someone with access to hardware and sufficient
time and motivation can try to get that working, but for now disk
on chip is not supported.

Signed-off-by: Scott Wood <scottwood@freescale.com>