* 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.