image: Move hurd image definition to a dedicated file.

This moves hurd-disk-image to a dedicated file. It also defines a default
operating-system so that the image can be built standalone.

* gnu/system/images/hurd.scm: New file,
* gnu/local.mk (GNU_SYSTEM_MODULES): add it,
* gnu/system/image.scm (root-offset, root-label): Export it,
(hurd-disk-image): remove it as this is now defined in the new, Hurd dedicated
file above,
(find-image): adapt to avoid loop dependency.