nandwrite: fix the bug of writing a yaffs2 image to NAND

nandwrite: fix the bug of writing a yaffs2 image to NAND

The tool mkyaffs2image doesn't know the oob layout of a NAND flash, so it
puts the yaffs2 tags at the offset 0 of oob area, as a result, the image
generated by mkyaffs2image is different with the image dumped by nanddump.
Now adding a parameter "-r" for nandwrite to differentiate these images.

Write a image generated by mkyaffs2image:
$> nandwrite -a -o /dev/mtd3 yaffs2.bin

Write a image dumped by nanddump:
$> nandwrite -a -r /dev/mtd3 image.bin

Signed-off-by: Stanley.Miao <stanley.miao@windriver.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>