[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>