ppc: Deprecate qemu-system-ppcemb

ppc: Deprecate qemu-system-ppcemb

qemu-system-ppcemb has been once split of qemu-system-ppc to support
CPU page sizes < 4096 for some of the embedded 4xx PowerPC CPUs.
However, there was hardly any OS available in the wild that really
used such small page sizes (Linux uses 4096 on PPC), so there is
no known recent use case for this separate build anymore. It's
rather cumbersome to maintain a separate set of config switches for
this, and it's wasting compile and test time of all the developers
who have to build all QEMU targets to verify that their changes did
not break anything.

Except for the small CPU page sizes, qemu-system-ppc can be used as
a full replacement for qemu-system-ppcemb since it contains all the
embedded 4xx PPC boards and CPUs, too. Thus let's start the deprecation
process for qemu-system-ppcemb to see whether somebody still needs
the small page sizes or whether we could finally remove this unloved
separate build.

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