system: Support the addition of old entries in the GRUB menu.

* gnu/system.scm (operating-system-grub.cfg): Add 'old-entries'
  parameter.  Pass it to 'grub-configuration-file'.
* gnu/system/grub.scm (grub-configuration-file): Add #:old-entries
  parameter.  Honor it.

gnu/system.scm

@@ -493,8 +493,9 @@ (define (kernel->grub-label kernel)
                  (package-version kernel)
                  " (technology preview)"))
 
-(define (operating-system-grub.cfg os)
-  "Return the GRUB configuration file for OS."
+(define* (operating-system-grub.cfg os #:optional (old-entries '()))
+  "Return the GRUB configuration file for OS.  Use OLD-ENTRIES to populate the
+\"old entries\" menu."
   (mlet* %store-monad
       ((system      (operating-system-derivation os))
        (root-fs ->  (operating-system-root-file-system os))
@@ -509,7 +510,8 @@ (define (operating-system-grub.cfg os)
                                   #~(string-append "--load=" #$system
                                                    "/boot")))
                            (initrd #~(string-append #$system "/initrd"))))))
-    (grub-configuration-file (operating-system-bootloader os) entries)))
+    (grub-configuration-file (operating-system-bootloader os) entries
+                             #:old-entries old-entries)))
 
 (define (operating-system-parameters-file os)
   "Return a file that describes the boot parameters of OS.  The primary use of

gnu/system/grub.scm

@@ -63,9 +63,12 @@ (define-record-type* <menu-entry>
   (initrd          menu-entry-initrd))     ; file name of the initrd as a gexp
 
 (define* (grub-configuration-file config entries
-                                  #:key (system (%current-system)))
+                                  #:key
+                                  (system (%current-system))
+                                  (old-entries '()))
   "Return the GRUB configuration file corresponding to CONFIG, a
-<grub-configuration> object."
+<grub-configuration> object.  OLD-ENTRIES is taken to be a list of menu
+entries corresponding to old generations of the system."
   (define all-entries
     (append entries (grub-configuration-menu-entries config)))
 
@@ -93,7 +96,14 @@ (define builder
                           (($ <menu-entry> _ linux)
                            linux))
                          all-entries))
-          #$@(map entry->gexp all-entries))))
+          #$@(map entry->gexp all-entries)
+
+          #$@(if (pair? old-entries)
+                 #~((format port "
+submenu \"GNU system, old configurations...\" {~%")
+                    #$@(map entry->gexp old-entries)
+                    (format port "}~%"))
+                 #~()))))
 
   (gexp->derivation "grub.cfg" builder))