powerpc/mpc8641hpcn: Move environment to avoid conflict

U-Boot on this board grew a long time ago past the 384 KiB that
it reserves for the U-Boot image, before the environment.  Thus,
saveenv overwrites the U-Boot image and bricks the board.

I tried to find out when U-Boot grew beyond this point, but there is a
long stretch in the history where this board did not build -- and
AFAICT when it did fit in 384 KiB, it was missing vital features such
as fdt support.  Turning off CONFIG_VIDEO was not enough to make it
fit.  Thus, I don't think we have any choice other than to move the
environment.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>