linux-initrd: Handle builtin modules with different module and file names.

Previously, modules we want were not normalized before comparing with
builtin-modules, which was normalized by file-name->module-name.

This made the code unable to find modules like nls_iso8859-1 (normalized to
nls_iso8859_1) and usb-storage (normalized to usb_storage) when they're built
into the kernel.

* gnu/system/linux-initrd.scm (flat-linux-module-directory) [modules-to-lookup]:
Normalize module name before comparing with builtin ones.

Change-Id: Ifb3ab298b8435c38040a6d58cdb53a3a46eb8ee1
Merges: #7514

gnu/system/linux-initrd.scm

@@ -154,7 +154,11 @@ MODULES and taken from LINUX."
                  '())))
 
             (define modules-to-lookup
-              (lset-difference string=? '#$modules builtin-modules))
+              (remove (lambda (module)
+                        (member (normalize-module-name module)
+                                ;; Normalized by `file-name->module-name'.
+                                builtin-modules))
+                      '#$modules))
 
             (define modules
               (let* ((lookup  (cut find-module-file module-dir <>))