bootloader: extlinux: Stop using dd binary.

* gnu/bootloader/extlinux.scm (dd): Remove it,
(install-extlinux): replace dd call by Guile I/O procedures.
* gnu/system/vm.scm (qemu-image): Add (ice-9 binary-ports) to used-modules
list to provide "get-bytevector-n" and "put-bytevector".
* guix/scripts/system.scm (bootloader-installer-derivation): Ditto.

gnu/bootloader/extlinux.scm

@@ -85,14 +85,6 @@ TIMEOUT ~a~%"
 ;;; Install procedures.
 ;;;
 
-(define dd
-  #~(lambda (bs count if of)
-      (zero? (system* "dd"
-                      (string-append "bs=" (number->string bs))
-                      (string-append "count=" (number->string count))
-                      (string-append "if=" if)
-                      (string-append "of=" of)))))
-
 (define (install-extlinux mbr)
   #~(lambda (bootloader device mount-point)
       (let ((extlinux (string-append bootloader "/sbin/extlinux"))
@@ -101,9 +93,15 @@ TIMEOUT ~a~%"
         (for-each (lambda (file)
                     (install-file file install-dir))
                   (find-files syslinux-dir "\\.c32$"))
-
+        (unless
-        (unless (and (zero? (system* extlinux "--install" install-dir))
+            (and (zero? (system* extlinux "--install" install-dir))
-                     (#$dd 440 1 (string-append syslinux-dir "/" #$mbr) device))
+                 (call-with-input-file (string-append syslinux-dir "/" #$mbr)
+                   (lambda (input)
+                     (let ((bv (get-bytevector-n input 440)))
+                       (call-with-output-file device
+                         (lambda (output)
+                           (put-bytevector output bv))
+                         #:binary #t)))))
           (error "failed to install SYSLINUX")))))
 
 (define install-extlinux-mbr

gnu/system/vm.scm

@@ -278,7 +278,8 @@ the image."
      #~(begin
          (use-modules (gnu build vm)
                       (guix build utils)
-                      (srfi srfi-26))
+                      (srfi srfi-26)
+                      (ice-9 binary-ports))
 
          (let ((inputs
                 '#$(append (list qemu parted e2fsprogs dosfstools)

guix/scripts/system.scm

@@ -676,7 +676,8 @@ and TARGET arguments."
     (gexp->file "bootloader-installer"
                 (with-imported-modules '((guix build utils))
                   #~(begin
-                      (use-modules (guix build utils))
+                      (use-modules (guix build utils)
+                                   (ice-9 binary-ports))
                       (#$installer #$bootloader #$device #$target))))))
 
 (define* (perform-action action os