system: grub: Change the default default-entry to 0.

* gnu/system/grub.scm (<grub-configuration>)[default-entry]: Change the
  default to 0.

gnu/system/grub.scm

@@ -49,7 +49,7 @@ (define-record-type* <grub-configuration>
   (menu-entries    grub-configuration-menu-entries   ; list
                    (default '()))
   (default-entry   grub-configuration-default-entry  ; integer
-                   (default 1))
+                   (default 0))
   (timeout         grub-configuration-timeout        ; integer
                    (default 5)))