arm: kirkwood: openrd: Increase U-Boot size in flash to make it fit

We have run now multiple times into size issues with the openrd
board port. To finally fix this, this patch now moves the U-Boot size
from 0x6.0000 to 0x8.0000, giving enough space for the next time.

This also changes the environment location and potentially the
MTD partitioning, but I see no better fix for now. Especially since
this board does not have an active maintainer.

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