order platforms with equal priority deterministically
diff --git a/aos/build/build.py b/aos/build/build.py
index e49f958..942e467 100755
--- a/aos/build/build.py
+++ b/aos/build/build.py
@@ -867,8 +867,8 @@
       build_env['PATH'] = os.environ['PATH']
     return build_env
 
-  sorted_platforms = sorted(platforms,
-                            key=lambda platform: -platform.priority())
+  sorted_platforms = sorted(
+      platforms, key=lambda platform: (-platform.priority(), str(platform)))
 
   to_build = []
   for platform in sorted_platforms: