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: I38da6bc20a5ceabf652ceeefb56ddd52b352d1ed

BoardConfig.mk

diff --git a/BoardConfig.mk b/BoardConfig.mk
index c07098f..6af551b 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -23,3 +23,5 @@
 WPA_SUPPLICANT_VERSION := VER_0_8_X
 
 USE_OPENGL_RENDERER := true
+
+BOARD_USE_LEGACY_UI := true