To build YAFFS in the Linux kernel tree you need to run the patch-ker.sh
script, giving the path to your kernel sources, e.g:

./patch-ker.sh /usr/src/linux

This will copy the yaffs files into fs/yaffs2 and modify the Kconfig
and Makefiles in the fs directory.

After you've run the script, go back to your normal kernel making procedure
and configure the yaffs settings you want.

Prolems? Contact the yaffs mailing list:

http://www.aleph1.co.uk/mailman/listinfo/yaffs.hmtl