system: Really filter out Linux device names for the store.

This is a followup to db4e8fd5d4.
Fixes <https://bugs.gnu.org/28445>.
Reported by Mark H Weaver <mhw@netris.org>.

* gnu/system.scm (ensure-not-/dev): New procedure.
(read-boot-parameters)[ensure-not-/dev]: Remove.
(operating-system-boot-parameters): Use it.

gnu/system.scm

@@ -231,6 +231,14 @@ directly by the user."
   (kernel-arguments boot-parameters-kernel-arguments)
   (initrd           boot-parameters-initrd))
 
+(define (ensure-not-/dev device)
+  "If DEVICE starts with a slash, return #f.  This is meant to filter out
+Linux device names such as /dev/sda, and to preserve GRUB device names and
+file system labels."
+  (if (and (string? device) (string-prefix? "/" device))
+      #f
+      device))
+
 (define (read-boot-parameters port)
   "Read boot parameters from PORT and return the corresponding
 <boot-parameters> object or #f if the format is unrecognized."
@@ -243,11 +251,6 @@ directly by the user."
       ((? string? device)
        device)))
 
-  (define (ensure-not-/dev device)
-    (if (and (string? device) (string-prefix? "/" device))
-        #f
-        device))
-
   (match (read port)
     (('boot-parameters ('version 0)
                        ('label label) ('root-device root)
@@ -939,7 +942,7 @@ kernel arguments for that derivation to <boot-parameters>."
                 (operating-system-user-kernel-arguments os)))
              (initrd initrd)
              (bootloader-name bootloader-name)
-             (store-device (fs->boot-device store))
+             (store-device (ensure-not-/dev (fs->boot-device store)))
              (store-mount-point (file-system-mount-point store))))))
 
 (define (device->sexp device)