Fixes image build error:

ln: accessing `arch/powerpc/boot/zImage': No such file or directory
make[1]: *** [arch/powerpc/boot/zImage] Error 1
make: *** [zImage] Error 2
Signed-off-by: NSegher Boessenkool <segher@kernel.crashing.org>
Acked-by: NG. Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: NKumar Gala <galak@kernel.crashing.org>