scripts: system: Accept <image> records as input.

scripts: system: Accept <image> records as input.

* guix/scripts/system.scm (system-derivation-for-action): Replace "os"
argument by "image". Remove "image-size", "image-type", "label" and
"volatile-root?"  arguments.
(perform-action): Ditto.
(process-action): Construct the <image> record and pass it to "perform-action"
procedure.
* tests/guix-system.sh: Adapt accordingly.
* gnu/system/images/hurd.scm: Return the default image.
* gnu/system/images/novena.scm: Ditto.
* gnu/system/images/pine64.scm: Ditto.
* gnu/system/images/pinebook-pro.scm Ditto.