image: Add 'target' support.

* gnu/image.scm (<image>)[target]: New field,
(image-target): new public method.
* gnu/system/image.scm (hurd-disk-image): Set "i586-pc-gnu" as image 'target'
field,
(maybe-with-target): new procedure,
(system-image): honor image 'target' field using the above procedure.

gnu/image.scm

@@ -33,6 +33,7 @@ (define-module (gnu image)
             image
             image-name
             image-format
+            image-target
             image-size
             image-operating-system
             image-partitions
@@ -67,6 +68,8 @@ (define-record-type* <image>
   image make-image
   image?
   (format             image-format) ;symbol
+  (target             image-target
+                      (default #f))
   (size               image-size  ;size in bytes as integer
                       (default 'guess))
   (operating-system   image-operating-system  ;<operating-system>

gnu/system/image.scm

@@ -104,6 +104,7 @@ (define hurd-initialize-root-partition
 (define hurd-disk-image
   (image
    (format 'disk-image)
+   (target "i586-pc-gnu")
    (partitions
     (list (partition
            (size 'guess)
@@ -519,6 +520,14 @@ (define (root-uuid os)
                             (type root-file-system-type))
                           file-systems-to-keep)))))
 
+(define-syntax-rule (maybe-with-target image exp ...)
+  (let ((target (image-target image)))
+    (if target
+        (with-parameters ((%current-target-system target))
+          exp ...)
+        (begin
+          exp ...))))
+
 (define* (system-image image)
   "Return the derivation of IMAGE.  It can be a raw disk-image or an ISO9660
 image, depending on IMAGE format."
@@ -530,6 +539,7 @@ (define substitutable? (image-substitutable? image))
          (bootcfg (operating-system-bootcfg os))
          (bootloader (bootloader-configuration-bootloader
                       (operating-system-bootloader os))))
+    (maybe-with-target image
       (case (image-format image)
         ((disk-image)
          (system-disk-image image*
@@ -555,7 +565,7 @@ (define substitutable? (image-substitutable? image))
           ;; This happens if some limits are exceeded, see:
           ;; https://lists.gnu.org/archive/html/grub-devel/2020-06/msg00048.html
           #:grub-mkrescue-environment
-        '(("MKRESCUE_SED_MODE" . "mbr_only")))))))
+          '(("MKRESCUE_SED_MODE" . "mbr_only"))))))))
 
 (define (find-image file-system-type target)
   "Find and return an image built that could match the given FILE-SYSTEM-TYPE,
@@ -570,4 +580,8 @@ (define (find-image file-system-type target)
         (else
          efi-disk-image)))))
 
+;;; Local Variables:
+;;; eval: (put 'maybe-with-target 'scheme-indent-function 1)
+;;; End:
+
 ;;; image.scm ends here