system: image: Fix root offset on rockchip platforms.

Fixes: <https://issues.guix.gnu.org/45584>.

* gnu/system/image.scm (arm32-disk-image, arm64-disk-image): Turn into
procedures, taking the root partition offset as argument.
* gnu/system/images/novena.scm (novena-image-type): Adapt accordingly.
* gnu/system/images/pine64.scm (pine64-image-type): Ditto.
* gnu/system/images/pinebook-pro.scm (pinebook-pro-image-type): Use a 9MiB
offset for the root partition.

gnu/system/image.scm

@@ -128,21 +128,21 @@ (define iso9660-image
            (label "GUIX_IMAGE")
            (flags '(boot)))))))
 
-(define arm32-disk-image
+(define* (arm32-disk-image #:optional (offset root-offset))
   (image
    (format 'disk-image)
    (target "arm-linux-gnueabihf")
    (partitions
     (list (partition
            (inherit root-partition)
-           (offset root-offset))))
+           (offset offset))))
    ;; FIXME: Deleting and creating "/var/run" and "/tmp" on the overlayfs
    ;; fails.
    (volatile-root? #f)))
 
-(define arm64-disk-image
+(define* (arm64-disk-image #:optional (offset root-offset))
   (image
-   (inherit arm32-disk-image)
+   (inherit (arm32-disk-image offset))
    (target "aarch64-linux-gnu")))
 
 
@@ -189,12 +189,12 @@ (define uncompressed-iso-image-type
 (define arm32-image-type
   (image-type
    (name 'arm32-raw)
-   (constructor (cut image-with-os arm32-disk-image <>))))
+   (constructor (cut image-with-os (arm32-disk-image) <>))))
 
 (define arm64-image-type
   (image-type
    (name 'arm64-raw)
-   (constructor (cut image-with-os arm64-disk-image <>))))
+   (constructor (cut image-with-os (arm64-disk-image) <>))))
 
 
 ;;

gnu/system/images/novena.scm

@@ -52,7 +52,7 @@ (define novena-barebones-os
 (define novena-image-type
   (image-type
    (name 'novena-raw)
-   (constructor (cut image-with-os arm32-disk-image <>))))
+   (constructor (cut image-with-os (arm32-disk-image) <>))))
 
 (define novena-barebones-raw-image
   (image

gnu/system/images/pine64.scm

@@ -57,7 +57,7 @@ (define pine64-barebones-os
 (define pine64-image-type
   (image-type
    (name 'pine64-raw)
-   (constructor (cut image-with-os arm64-disk-image <>))))
+   (constructor (cut image-with-os (arm64-disk-image) <>))))
 
 (define pine64-barebones-raw-image
   (image

gnu/system/images/pinebook-pro.scm

@@ -57,7 +57,9 @@ (define pinebook-pro-barebones-os
 (define pinebook-pro-image-type
   (image-type
    (name 'pinebook-pro-raw)
-   (constructor (cut image-with-os arm64-disk-image <>))))
+   (constructor (cut image-with-os
+                     (arm64-disk-image (* 9 (expt 2 20))) ;9MiB
+                     <>))))
 
 (define pinebook-pro-barebones-raw-image
   (image