bootstd: Move bootflow-clearing to bootstd

This relates to more than just the bootdev, since there is a global list
of bootflows. Move the function to the bootstd file and rename it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/cmd/bootflow.c b/cmd/bootflow.c
index 8962464..1c1146c 100644
--- a/cmd/bootflow.c
+++ b/cmd/bootflow.c
@@ -197,7 +197,7 @@
 		show_header();
 	}
 	if (dev)
-		bootdev_clear_bootflows(dev);
+		bootstd_clear_bootflows_for_bootdev(dev);
 	else
 		bootstd_clear_glob();
 	for (i = 0,