hw/ppc/Makefile: Add a way to disable the PPC4xx boards

hw/ppc/Makefile: Add a way to disable the PPC4xx boards

We've got the config switch CONFIG_PPC4XX, so we should use it
in the Makefile accordingly and only include the PPC4xx boards
if this switch has been enabled. (Note: Unfortunately, the files
ppc4xx_devs.c and ppc405_uc.c still have to be included in the
build anyway to fulfil some complicated linker dependencies ...
so these are subject to a more thourough clean-up later)

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>