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/bootdev-uclass.c b/boot/bootdev-uclass.c
index 5683006..13ac69e 100644
--- a/boot/bootdev-uclass.c
+++ b/boot/bootdev-uclass.c
@@ -36,7 +36,6 @@
 
 int bootdev_add_bootflow(struct bootflow *bflow)
 {
-	struct bootdev_uc_plat *ucp = dev_get_uclass_plat(bflow->dev);
 	struct bootstd_priv *std;
 	struct bootflow *new;
 	int ret;
@@ -52,7 +51,11 @@
 	memcpy(new, bflow, sizeof(*bflow));
 
 	list_add_tail(&new->glob_node, &std->glob_head);
-	list_add_tail(&new->bm_node, &ucp->bootflow_head);
+	if (bflow->dev) {
+		struct bootdev_uc_plat *ucp = dev_get_uclass_plat(bflow->dev);
+
+		list_add_tail(&new->bm_node, &ucp->bootflow_head);
+	}
 
 	return 0;
 }
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);