ARM: kirkwood: Enforce size limit for guruplug

The u-boot binary sits in flash immediately before the environment.
Don't allow the binary size to grow into the environment space.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>