Make the operating-system field mandatory as creating an image without it
makes no sense. Introduce a new macro, image-without-os for the specific cases
where the image is only created to be inherited from afterwards.
* gnu/image.scm (<image>)[operating-system]: Make it mandatory.
* gnu/system/image.scm (image-without-os): New macro.
(efi-disk-image, efi32-disk-image, iso9660-image, docker-image,
raw-with-offset-disk-image): Use it.
* gnu/system/images/hurd.scm (hurd-disk-image): Ditto.
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