[Yaffs] CVS tweak of configuration options.

Ideally, it should be possible to build yaffs2 for either 512-byte NAND
devices or 2k-or-bigger NAND devices, but right now the code isn't
really set up to do that.

For the moment, I've changed the Kconfig file so that it always selects
both, and added "Fix that" to my TODO list.

I've also made a few small changes to the new modulesconfig.h file to
add documentation and to make it match the defaults set up by Kconfig.

Marty