gnu: Refactor boot-service-type and activation-service-type.

* gnu/services.scm (boot-service-type) <compose>: Use the "identity"
  procedure instead of the "append" procedure because it more accurately
  reflects the intent, which is to simply return the single list of
  extensions to which fold-services applies the "compose" procedure.
  (activation-service-type) <compose>: Likewise.

gnu/services.scm

@@ -352,7 +352,7 @@ directory."
                 (extensions
                  (list (service-extension system-service-type
                                           boot-script-entry)))
-                (compose append)
+                (compose identity)
                 (extend compute-boot-script)))
 
 (define %boot-service
@@ -457,7 +457,7 @@ ACTIVATION-SCRIPT-TYPE."
                 (extensions
                  (list (service-extension boot-service-type
                                           gexps->activation-gexp)))
-                (compose append)
+                (compose identity)
                 (extend second-argument)))
 
 (define %activation-service