zenfone7: replace deprecated sepolicy BOARD_PLAT_ by SYSTEM_EXT_

Change-Id: Ia11bcf85b45dd8708b4a661956ca54ffedef7eff
diff --git a/BoardConfig.mk b/BoardConfig.mk
index 3976b84..e2939c4 100755
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -41,7 +41,7 @@
 TARGET_SYSTEM_PROP += $(DEVICE_PATH)/system.prop
 
 # Sepolicy
-BOARD_PLAT_PRIVATE_SEPOLICY_DIR += $(DEVICE_PATH)/sepolicy/private
+SYSTEM_EXT_PRIVATE_SEPOLICY_DIRS += $(DEVICE_PATH)/sepolicy/private
 
 # inherit from the proprietary version
 -include vendor/asus/zenfone7/BoardConfigVendor.mk