Add busybox shell replacements module

Add busybox shell replacements module

On the OLPC XO-1, there is a noticable delay during boot while the
initramfs is loaded from disk and uncompressed, so we have an interest
in making it small. We are also pushed for disk space.

Using busybox instead of all the regular tools saves a lot of space.
I have not tried every module but the basics are working with busybox's
replacements. Our initramfs is now down to 1.9mb.