les: implement ultralight client (#16904)

For more information about this light client mode, read
https://hackmd.io/s/HJy7jjZpm

cmd/geth/config.go

@@ -125,6 +125,7 @@ func makeConfigNode(ctx *cli.Context) (*node.Node, gethConfig) {
 	}
 
 	// Apply flags.
+	utils.SetULC(ctx, &cfg.Eth)
 	utils.SetNodeConfig(ctx, &cfg.Node)
 	stack, err := node.New(&cfg.Node)
 	if err != nil {