At present the Sam460ex board is activated by the general CONFIG_PPC4XX
option. However that includes the board for both ppc-softmmu and
(deprecated) ppcemb-softmmu builds. As Sam460ex is developed, that would
require adding more things into ppcemb-softmmu, which we don't want to do.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>