Allow other layer to append to PRODUCT_*_SEPOLICY_DIR

- Car products need to append them

Bug: 155492686
Test: Check generated sepolicy in car products
Change-Id: Idc40cfed38a29be012e60ed635924d514843d952
diff --git a/shared/BoardConfig.mk b/shared/BoardConfig.mk
index 2e1a863..c9d09ba 100644
--- a/shared/BoardConfig.mk
+++ b/shared/BoardConfig.mk
@@ -104,12 +104,12 @@
 # vendor sepolicy
 BOARD_VENDOR_SEPOLICY_DIRS += device/google/cuttlefish/shared/sepolicy/vendor
 BOARD_VENDOR_SEPOLICY_DIRS += device/google/cuttlefish/shared/sepolicy/vendor/google
-# product sepolicy
-PRODUCT_PRIVATE_SEPOLICY_DIRS := device/google/cuttlefish/shared/sepolicy/product/private
-# PRODUCT_PUBLIC_SEPOLICY_DIRS := device/google/cuttlefish/shared/sepolicy/product/public
+# product sepolicy, allow other layers to append
+PRODUCT_PRIVATE_SEPOLICY_DIRS += device/google/cuttlefish/shared/sepolicy/product/private
+# PRODUCT_PUBLIC_SEPOLICY_DIRS += device/google/cuttlefish/shared/sepolicy/product/public
 # system_ext sepolicy
-BOARD_PLAT_PRIVATE_SEPOLICY_DIR := device/google/cuttlefish/shared/sepolicy/system_ext/private
-# BOARD_PLAT_PUBLIC_SEPOLICY_DIR := device/google/cuttlefish/shared/sepolicy/system_ext/public
+BOARD_PLAT_PRIVATE_SEPOLICY_DIR += device/google/cuttlefish/shared/sepolicy/system_ext/private
+# BOARD_PLAT_PUBLIC_SEPOLICY_DIR += device/google/cuttlefish/shared/sepolicy/system_ext/public
 
 VSOC_STLPORT_INCLUDES :=
 VSOC_STLPORT_LIBS :=