[Yaffs] Change to tags compatability

[Charles Manning]

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