gnu: image: Add support for unformatted partitions.

* gnu/build/image.scm (make-unformatted-image): New procedure.
(make-partition-image): Add support for unformatted partition.
* gnu/system/image.scm (system-disk-image)[partition->gpt-type]: Add
case for using unformatted partition uuid.

gnu/build/image.scm

@@ -7,6 +7,7 @@
 ;;; Copyright © 2020 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;; Copyright © 2022 Pavel Shlyak <p.shlyak@pantherx.org>
 ;;; Copyright © 2022 Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>
+;;; Copyright © 2023 Efraim Flashner <efraim@flashner.co.il>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -122,6 +123,14 @@ turn doesn't take any constant overhead into account, force a 1-MiB minimum."
                           (string-append "::" file))))
               (scandir root))))
 
+(define* (make-unformatted-image partition target)
+  "Make an unformatted partition of a certain size."
+  (let ((size (partition-size partition)))
+    ;; Create the file and then truncate it to the desired size.
+    (with-output-to-file target
+      (lambda _ (display "")))
+    (truncate-file target size)))
+
 (define* (make-partition-image partition-sexp target root)
   "Create and return the image of PARTITION-SEXP as TARGET.  Use the given
 ROOT directory to populate the image."
@@ -134,6 +143,8 @@ ROOT directory to populate the image."
       (make-vfat-image partition target root 16))
      ((string=? type "fat32")
       (make-vfat-image partition target root 32))
+     ((string=? type "unformatted")
+      (make-unformatted-image partition target))
      (else
       (raise (condition
               (&message

gnu/system/image.scm

@@ -4,6 +4,7 @@
 ;;; Copyright © 2022 Pavel Shlyak <p.shlyak@pantherx.org>
 ;;; Copyright © 2022 Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>
 ;;; Copyright © 2022 Alex Griffin <a@ajgrf.com>
+;;; Copyright © 2023 Efraim Flashner <efraim@flashner.co.il>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -390,6 +391,9 @@ used in the image."
          ((or (string=? file-system "vfat")
               (string=? file-system "fat16")
               (string=? file-system "fat32")) "F")
+         ((and (string=? file-system "unformatted")
+               (partition-uuid partition))
+          (uuid->string (partition-uuid partition)))
          (else
           (raise (condition
                   (&message