system: Add 'label' field to <operating-system>.

* gnu/system.scm (<operating-system>)[label]: New field.
(operating-system-default-label): New procedure.
(operating-system-boot-parameters): Use it instead of
'kernel->boot-label'.
* doc/guix.texi (operating-system Reference): Document it.

doc/guix.texi

@@ -10436,6 +10436,10 @@ the command-line of the kernel---e.g., @code{("console=ttyS0")}.
 @item @code{bootloader}
 The system bootloader configuration object.  @xref{Bootloader Configuration}.
 
+@item @code{label}
+This is the label (a string) as it appears in the bootloader's menu entry.
+The default label includes the kernel name and version.
+
 @item @code{keyboard-layout} (default: @code{#f})
 This field specifies the keyboard layout to use in the console.  It can be
 either @code{#f}, in which case the default keyboard layout is used (usually

gnu/system.scm

@@ -78,6 +78,8 @@
             operating-system-kernel
             operating-system-kernel-file
             operating-system-kernel-arguments
+            operating-system-label
+            operating-system-default-label
             operating-system-initrd-modules
             operating-system-initrd
             operating-system-users
@@ -160,6 +162,9 @@
   (kernel-arguments operating-system-user-kernel-arguments
                     (default '()))                ; list of gexps/strings
   (bootloader operating-system-bootloader)        ; <bootloader-configuration>
+  (label operating-system-label                   ; string
+         (thunked)
+         (default (operating-system-default-label this-operating-system)))
 
   (keyboard-layout operating-system-keyboard-layout ;#f | <keyboard-layout>
                    (default #f))
@@ -918,6 +923,11 @@ listed in OS.  The C library expects to find it under
                         (inferior-package-version kernel)))
         (else "GNU")))
 
+(define (operating-system-default-label os)
+  "Return the default label for OS, as it will appear in the bootloader menu
+entry."
+  (kernel->boot-label (operating-system-kernel os)))
+
 (define (store-file-system file-systems)
   "Return the file system object among FILE-SYSTEMS that contains the store."
   (match (filter (lambda (fs)
@@ -966,7 +976,7 @@ such as '--root' and '--load' to <boot-parameters>."
          (bootloader      (bootloader-configuration-bootloader
                            (operating-system-bootloader os)))
          (bootloader-name (bootloader-name bootloader))
-         (label           (kernel->boot-label (operating-system-kernel os))))
+         (label           (operating-system-label os)))
     (boot-parameters
      (label label)
      (root-device root-device)