core/linux-kirkwood-dt: Build devicetree blob for the Zyxel NSA320

Upstream kernel source now contains a dts file for this device, but
the dtb won't get built by the 'dtbs' target, it has to be specified
explicitly.
Merge pull request #941 from PlanetEater/pr/linux-kirkwood-dt

core/linux-kirkwood-dt: Build devicetree blob for the Zyxel NSA320