[POWERPC] Update to linkstation / kurobox support

Linkstation systems capable of running mainline kernels use u-boot as a
bootloader, so, specifying a suitable kernel command is not a problem.
Don't guess. Also extend linkstation_defconfig to support the linkstation HS
model with a IT8212 IDE controller and kuroboxHD/HD-HLAN linkstation
models with a tulip ethernet chip. The latter also require a slightly
different .dts file, which is also included with this patch.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>