[POWERPC] Select u-image as default image for Linkstation

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: Segher Boessenkool <segher@kernel.crashing.org>
Acked-by: G. Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>