bootstd: Allow the bootdev to be optional in bootflows

With global bootmeths we want to scan without a bootdev. Update the logic
to allow this.

Change the bootflow command to show the bootdev only when valid.

Signed-off-by: Simon Glass <sjg@chromium.org>

boot/bootflow.c

@@ -307,7 +307,8 @@ void bootflow_free(struct bootflow *bflow)
 
 void bootflow_remove(struct bootflow *bflow)
 {
-	list_del(&bflow->bm_node);
+	if (bflow->dev)
+		list_del(&bflow->bm_node);
 	list_del(&bflow->glob_node);
 
 	bootflow_free(bflow);