env: Move set_default_vars to env.h

Move this function to the new header file and rename it so it has an env_
prefix.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

cmd/nvedit.c

@@ -820,7 +820,7 @@ static int do_env_default(cmd_tbl_t *cmdtp, int flag,
 	}
 	if (!all && (argc > 0)) {
 		/* Reset individual variables */
-		set_default_vars(argc, argv, env_flag);
+		env_set_default_vars(argc, argv, env_flag);
 		return 0;
 	}