* guix/scripts/system.scm (%options): Add `--label'.
(system-derivation-for-action): Take a #:label key to set volume ID.
(perform-action): Take a #:label key.
(%default-options): Add default label value.
(process-action): Pass label value from command-line to perform-action.
* gnu/system/image.scm (image-with-label): New procedure.