bootstd: Add a way to set up a bootflow

Add a function to init a bootflow, to reduce code duplication.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/bootdev-uclass.c b/boot/bootdev-uclass.c
index 9d98bee..affe0d3 100644
--- a/boot/bootdev-uclass.c
+++ b/boot/bootdev-uclass.c
@@ -440,10 +440,7 @@
 
 	if (!ops->get_bootflow)
 		return -ENOSYS;
-	memset(bflow, '\0', sizeof(*bflow));
-	bflow->dev = dev;
-	bflow->method = iter->method;
-	bflow->state = BOOTFLOWST_BASE;
+	bootflow_init(bflow, dev, iter->method);
 
 	return ops->get_bootflow(dev, iter, bflow);
 }