bootstd: Avoid calling unavailable block functions

When BLK is not enabled but BOOTSTD is, some features of standard boot
become unavailable. Add a check for this in the only site that is
currently apparent.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/bootdev-uclass.c b/boot/bootdev-uclass.c
index 807f8df..64ec4fd 100644
--- a/boot/bootdev-uclass.c
+++ b/boot/bootdev-uclass.c
@@ -434,6 +434,9 @@
 	struct uclass *uc;
 	enum uclass_id id;
 
+	if (!CONFIG_IS_ENABLED(BLK))
+		return -ENOSYS;
+
 	ret = label_to_uclass(label, &seq, &method_flags);
 	if (ret < 0)
 		return log_msg_ret("uc", ret);