gnu: Add (gnu system).

* gnu/system/vm.scm (lower-inputs): Move to monads.scm.
  (qemu-image): Don't add GRUB-CONFIGURATION to the INPUTS-TO-COPY.
  (union, file-union, etc-directory): Move to gnu/system.scm.
  (%demo-operating-system): New variable.
  (system-qemu-image): Add 'os' parameter.  Rewrite in terms of
  'operating-system-derivation'.
* guix/monads.scm (lower-inputs): New procedure.
* gnu/system/grub.scm (grub-configuration-file): Change 'entries' to be
  a plain list instead of a list of monadic values.
* gnu/system.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.

guix/monads.scm

@@ -56,7 +56,8 @@
             package-file
             package->derivation
             built-derivations
-            derivation-expression))
+            derivation-expression
+            lower-inputs))
 
 ;;; Commentary:
 ;;;
@@ -319,6 +320,22 @@ OUTPUT directory of PACKAGE."
           (string-append out "/" file)
           out))))
 
+(define (lower-inputs inputs)
+  "Turn any package from INPUTS into a derivation; return the corresponding
+input list as a monadic value."
+  ;; XXX: Should probably be in (guix packages).
+  (with-monad %store-monad
+    (sequence %store-monad
+              (map (match-lambda
+                    ((name (? package? package) sub-drv ...)
+                     (mlet %store-monad ((drv (package->derivation package)))
+                       (return `(,name ,drv ,@sub-drv))))
+                    ((name (? string? file))
+                     (return `(,name ,file)))
+                    (tuple
+                     (return tuple)))
+                   inputs))))
+
 (define derivation-expression
   (store-lift build-expression->derivation))