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>
diff --git a/boot/bootflow.c b/boot/bootflow.c
index 24ba3c3..37bccb8 100644
--- a/boot/bootflow.c
+++ b/boot/bootflow.c
@@ -307,7 +307,8 @@
 
 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);