[PATCH] powerpc: Add missing vmlinux.bin target

With this patch 'make vmlinux.bin' works.  This is needed by
some embedded platforms.  Kumar already added the routines
to actually build the image in arch/powerpc/boot/Makefile.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>