ppc4xx: Fix problems in some ppc4xx board Makefiles

ppc4xx: Fix problems in some ppc4xx board Makefiles

Some 4xx Makefiles didn't add $(SOBJ) to their board library. This was
no till now problem, since those boards included this object (init.o
most of the time) directly from their linker scripts. This patch clean
this up, so that all objects are now collected in the board library. This
is in preparation for the upcoming PPC4xx linker script consolidation.

Signed-off-by: Stefan Roese <sr@denx.de>