[POWERPC] Fix zImage-dtb.initrd build error

The pattern substitution rules were failing when used with zImage-dtb
targets.  If zImage-dtb.initrd was selected, the pattern substitution
would generate "zImage.initrd-dtb" instead of "zImage-dtb.initrd" which
caused the build to fail.

This renames zImage-dtb to dtbImage to avoid the problem entirely.
By not using the zImage prefix then is no potential for namespace
collisions.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index 1c6ce35..ab5cfe8 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -155,7 +155,7 @@
 
 CPPFLAGS_vmlinux.lds	:= -Upowerpc
 
-BOOT_TARGETS = zImage zImage.initrd uImage treeImage.% cuImage.%
+BOOT_TARGETS = zImage zImage.initrd uImage zImage% dtbImage% treeImage.% cuImage.%
 
 PHONY += $(BOOT_TARGETS)