gnu: linux-libre: Move info manual to doc output.

The kernel Linux is installed in the system profile, which means the manual
was not easily discoverable.  Move it to the "doc" output, so that users can
install the Linux documentation explicitly in their user profile without
pulling the whole Linux package.

* gnu/packages/linux.scm (make-linux-libre*) [outputs]: Conditionally add a
"doc" output.
[phases] {install-doc}: Install the info manual to the doc output, an create a
"linux" symlink to it for convenience.
* guix/build-system/linux-module.scm (make-linux-module-builder): Delete the
"doc" output, if present.

gnu/packages/linux.scm

@@ -816,6 +816,7 @@ (define* (make-linux-libre* version gnu-revision source supported-systems
     (version version)
     (source source)
     (supported-systems supported-systems)
+    (outputs `("out" ,@(if build-doc? '("doc") '())))
     (build-system gnu-build-system)
     (arguments
      (list
@@ -842,10 +843,16 @@ (define* (make-linux-libre* version gnu-revision source supported-systems
                         (invoke "make" "infodocs")))
                     (add-after 'build-doc 'install-doc
                       (lambda _
-                        (with-directory-excursion "Documentation/output"
-                          (invoke "make" "-C" "texinfo" "install-info"
-                                  (string-append "infodir=" #$output
-                                                 "/share/info"))))))
+                        (let* ((info-dir (string-append #$output:doc
+                                                        "/share/info"))
+                               (info (string-append
+                                      info-dir "/TheLinuxKernel.info.gz")))
+                          (with-directory-excursion "Documentation/output"
+                            (invoke "make" "-C" "texinfo" "install-info"
+                                    (string-append "infodir=" info-dir)))
+                          ;; Create a symlink, for convenience.
+                          (symlink info (string-append info-dir
+                                                       "/linux.info.gz"))))))
                  #~())
           (add-before 'configure 'set-environment
             (lambda* (#:key target #:allow-other-keys)

guix/build-system/linux-module.scm

@@ -59,6 +59,7 @@ (define (make-linux-module-builder linux)
     (name (string-append (package-name linux) "-module-builder"))
     (inputs
      `(("linux" ,linux)))
+    (outputs (delete "doc" (package-outputs linux)))
     (arguments
      (substitute-keyword-arguments (package-arguments linux)
        ((#:phases phases)