dfu, nand: before write a buffer to nand, erase the nand sectors

dfu, nand: before write a buffer to nand, erase the nand sectors

before writing the received buffer to nand, erase the nand
sectors. If not doing this, nand write fails. See for
more info here:

http://lists.denx.de/pipermail/u-boot/2013-June/156361.html

Using the nand erase option "spread", maybe overwrite
blocks on, for example another mtd partition, if the
erasing range contains bad blocks.
So a limit option is added to nand_erase_opts()

Signed-off-by: Heiko Schocher <[email protected]>
Cc: Scott Wood <[email protected]>
Cc: Pantelis Antoniou <[email protected]>
Cc: Lukasz Majewski <[email protected]>
Cc: Kyungmin Park <[email protected]>
Cc: Marek Vasut <[email protected]>
Cc: Tom Rini <[email protected]>
Signed-off-by: Scott Wood <[email protected]>