Use build directory for app build artifacts by default

This avoids poluting peoples workspace

Change-Id: I3264f5be7d10f562f40124e6b2eeb4b141715b5a
diff --git a/tools/run_on_app.py b/tools/run_on_app.py
index bab9b32..2379912 100755
--- a/tools/run_on_app.py
+++ b/tools/run_on_app.py
@@ -40,7 +40,7 @@
                     choices=TYPES)
   result.add_option('--out',
                     help='Where to place the output',
-                    default=os.getcwd())
+                    default=utils.BUILD)
   result.add_option('--no-build',
                     help='Run without building first',
                     default=False,