image: Make the operating-system field mandatory.

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.

gnu/image.scm

@@ -170,8 +170,7 @@ that is not in SET, mentioning FIELD in the error message."
   (size               image-size  ;size in bytes as integer
                       (default 'guess)
                       (sanitize validate-size))
-  (operating-system   image-operating-system  ;<operating-system>
-                      (default #f))
+  (operating-system   image-operating-system)  ;<operating-system>
   (partition-table-type image-partition-table-type ; 'mbr or 'gpt
                       (default 'mbr)
                       (sanitize validate-partition-table-type))

gnu/system/image.scm

@@ -65,6 +65,7 @@
   #:use-module (ice-9 match)
   #:export (root-offset
             root-label
+            image-without-os
 
             esp-partition
             esp32-partition
@@ -102,6 +103,12 @@
 ;; Generic root partition label.
 (define root-label "Guix_image")
 
+(define-syntax-rule (image-without-os . fields)
+    "Return an image record with the mandatory operating-system field set to
+#false.  This is useful when creating an image record that will serve as a
+parent image record."
+   (image (operating-system #false) . fields))
+
 (define esp-partition
   (partition
    (size (* 40 (expt 2 20)))
@@ -127,17 +134,17 @@
    (initializer (gexp initialize-root-partition))))
 
 (define efi-disk-image
-  (image
+  (image-without-os
    (format 'disk-image)
    (partitions (list esp-partition root-partition))))
 
 (define efi32-disk-image
-  (image
+  (image-without-os
    (format 'disk-image)
    (partitions (list esp32-partition root-partition))))
 
 (define iso9660-image
-  (image
+  (image-without-os
    (format 'iso9660)
    (partitions
     (list (partition
@@ -146,11 +153,11 @@
            (flags '(boot)))))))
 
 (define docker-image
-  (image
+  (image-without-os
    (format 'docker)))
 
 (define* (raw-with-offset-disk-image #:optional (offset root-offset))
-  (image
+  (image-without-os
    (format 'disk-image)
    (partitions
     (list (partition

gnu/system/images/hurd.scm

@@ -74,7 +74,7 @@
                            #:wal-mode? #f)))))
 
 (define hurd-disk-image
-  (image
+  (image-without-os
    (format 'disk-image)
    (platform hurd)
    (partitions