configs/sheevaplug: added a few additional commands

configs/sheevaplug: added a few additional commands

This patch includes a few additional commands in the sheevaplug
version of u-boot:
- support for LONGHELP so you can get help messages
- auto completion and command editing
- ubi and mii support
- ext2 filesystem (convenient if you have an ext2 from which you want to boot)
- jffs2 and ubifs filesystems (if you want to use these in NAND)

This also makes it more similar to openrd client.

Side effect of this patch is that the code now needs 3 sectors i.s.o. 2
so an existing env is overwritten

Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>