Allow appending PRODUCT_*_SEPOLICY_DIRS

- Allow inherited devices to update the policy

Bug: 156009417
Test: build inherited car devices
Change-Id: I0660adf181cc5312269b95881d5a3603f17762e7
diff --git a/bonito-sepolicy.mk b/bonito-sepolicy.mk
index f618b8a..b63f345 100644
--- a/bonito-sepolicy.mk
+++ b/bonito-sepolicy.mk
@@ -1,5 +1,5 @@
-PRODUCT_PUBLIC_SEPOLICY_DIRS := device/google/bonito-sepolicy/public
-PRODUCT_PRIVATE_SEPOLICY_DIRS := device/google/bonito-sepolicy/private
+PRODUCT_PUBLIC_SEPOLICY_DIRS += device/google/bonito-sepolicy/public
+PRODUCT_PRIVATE_SEPOLICY_DIRS += device/google/bonito-sepolicy/private
 
 # vendors
 BOARD_VENDOR_SEPOLICY_DIRS += device/google/bonito-sepolicy/vendor/qcom/common