[Yaffs] Change to tags compatability

Hi YAFFSers

I just applied a change to CVS that I should have done a while ago.

Until now, YAFFS was marking bad blocks by writing 0x00 to the bad block
marker. This makes it harder for people to distinguish YAFFS-marked from
other bad blocks. I have changed this so that yaffs uses a 'Y' instead.

Bad block marking should not be an issue with good code, but is typically an
issue while people are ironing out their NAND interfacing.

-- Charles