services: static-networking-service: Wait for udev, except for loopback.

Reported by DusXMT on #guix.

* gnu/services/networking.scm (static-networking-service): Add
  'requirement' field.  Change 'documentation' to be a string literal.

gnu/services/networking.scm

@@ -50,9 +50,15 @@ (define* (static-networking-service interface ip
   (with-monad %store-monad
     (return
      (service
+
+      ;; Unless we're providing the loopback interface, wait for udev to be up
+      ;; and running so that INTERFACE is actually usable.
+      (requirement (if (memq 'loopback provision)
+                       '()
+                       '(udev)))
+
       (documentation
-       (string-append "Set up networking on the '" interface
-                      "' interface using a static IP address."))
+       "Bring up the networking interface using a static IP address.")
       (provision provision)
       (start #~(lambda _
                  ;; Return #t if successfully started.