arm: kirkwood: standardise the configuration

arm: kirkwood: standardise the configuration

The Marvell plugs are very similar systems, and so it makes sense
for their u-boots to have the same commands/configuration.

Add EXT4 and MII to Dreamplug, DATE to Guruplug and Sheevaplug.
Add CONFIG_SYS_ALT_MEMTEST to Sheevaplug.
There are still command differences around NAND, SPI/NOR.

Also default to building u-boot.kwb for Sheevaplug and Guruplug.

Signed-off-by: Quentin Armitage <quentin@armitage.org.uk>