PPChameleonEVB and CATcenter need a custom linker script

These boards use an embedded environment, which is not supported by the
generic arch/powerpc/cpu/ppc4xx/u-boot.lds script.

The breakage was introduced by commit 2cd95a25 "ppc4xx: Remove board
specific linker scripts from most PPC4xx boards"

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Andrea Marson <andrea.marson@dave-tech.it>
Acked-by: Stefan Roese <sr@denx.de>