define BOARD_USE_LEGACY_UI for PDK build

- coresponding Android.mk will be changed later to use legacy UI when
  this is set true.

Change-Id: I47a2a1d677656be246b64387ae8c470abce02ac9
diff --git a/BoardConfig.mk b/BoardConfig.mk
index 38c82b1..1d38da3 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -20,3 +20,4 @@
 
 USE_OPENGL_RENDERER := true
 
+BOARD_USE_LEGACY_UI := true