Force manifest booleans to have empty values.

Allowing values such as `<0>` or `"false"` be considered true has the
potential to cause confusion so only accept empty properties as booleans.

Change-Id: I8a9db6a97fc8687ac336138b7b8fc10eee69ae91
diff --git a/inc/hf/manifest.h b/inc/hf/manifest.h
index cfedc61..0e628e5 100644
--- a/inc/hf/manifest.h
+++ b/inc/hf/manifest.h
@@ -66,6 +66,7 @@
 	MANIFEST_ERROR_MALFORMED_INTEGER,
 	MANIFEST_ERROR_INTEGER_OVERFLOW,
 	MANIFEST_ERROR_MALFORMED_INTEGER_LIST,
+	MANIFEST_ERROR_MALFORMED_BOOLEAN,
 };
 
 enum manifest_return_code manifest_init(struct manifest *manifest,