Use concatenation versus assignment when making policy declarations

* Use concatenation versus assignment when making
  policy declarations inside BoardConfig.mk. This
  will allow sepolicy to exist in the vendor
  directory.

(cherryicked from b8318e8abfea1100558ac83693c51cd84edf7a97)

Change-Id: I0709fa4b8d6eaeac2fa11cd1b7106f459e443dc4
diff --git a/BoardConfig.mk b/BoardConfig.mk
index fb11e9b..6522ec0 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -94,11 +94,11 @@
 
 BOARD_HAL_STATIC_LIBRARIES := libdumpstate.hammerhead
 
-BOARD_SEPOLICY_DIRS := \
+BOARD_SEPOLICY_DIRS += \
        device/lge/hammerhead/sepolicy
 
 # The list below is order dependent
-BOARD_SEPOLICY_UNION := \
+BOARD_SEPOLICY_UNION += \
        device.te \
        app.te \
        file_contexts