build: image: Make partition uuid optional.

The uuid field of <partition> defaults to #false. This should be reflected
when creating the partition.

* gnu/build/image.scm (make-ext-image): Make it optional.

gnu/build/image.scm

@@ -86,7 +86,10 @@ turn doesn't take any constant overhead into account, force a 1-MiB minimum."
         (journal-options "lazy_itable_init=1,lazy_journal_init=1"))
     (apply invoke
            `("fakeroot" "mke2fs" "-t" ,fs "-d" ,root
-             "-L" ,label "-U" ,(uuid->string uuid)
+             "-L" ,label
+             ,@(if uuid
+                   `("-U" ,(uuid->string uuid))
+                   '())
              "-E" ,(format #f "root_owner=~a:~a,~a"
                            owner-uid owner-gid journal-options)
              ,@fs-options