Fix the bug that nandwrite can't write the yaffs2 image correctly

[Stanley.Miao]

Many people encountered this problerm, creating a image by mkyaffs2image,
then write it into a nand falsh with nandwrite, then mounted failed.

This is because mkyaffs2image didn't know the oob layout of a NAND flash,
so it put the yaffs2 tags at the offset 0 of oob area, nandwrite didn't
put it at right position when writing oobdata.

Stanley.