zenfone8: replace deprecated sepolicy BOARD_PLAT_ by SYSTEM_EXT_

Change-Id: I7f2f465ed36af031898a20ce6de5ce05457bee11
diff --git a/BoardConfig.mk b/BoardConfig.mk
index fd9027d..e909aa5 100755
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -208,7 +208,7 @@
 # Sepolicy
 include vendor/omni/sepolicy/sepolicy.mk
 include $(DEVICE_PATH)/sepolicy/SEPolicy.mk
-BOARD_PLAT_PRIVATE_SEPOLICY_DIR += $(DEVICE_PATH)/sepolicy/private \
+SYSTEM_EXT_PRIVATE_SEPOLICY_DIRS += $(DEVICE_PATH)/sepolicy/private \
                                    $(DEVICE_PATH)/sepolicy/qva/private
 
 PRODUCT_PRIVATE_SEPOLICY_DIRS += $(DEVICE_PATH)/sepolicy/product/private