bootstd: Detect empty bootmeth ordering

If the ordering produces no entries, this is an error. Report it, so that
the caller doesn't try to continue with a NULL bootmeth.

This fixes a crash in the bootflow_iter test when running with the sandbox
'default' device tree, instead of the required 'test' one.

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

boot/bootmeth-uclass.c

@@ -114,6 +114,8 @@ int bootmeth_setup_iter_order(struct bootflow_iter *iter)
 		}
 		count = upto;
 	}
+	if (!count)
+		return log_msg_ret("count2", -ENOENT);
 
 	iter->method_order = order;
 	iter->num_methods = count;