build_image.py recognize BOARD_PRODUCTIMAGE_PARTITION_RESERVED_SIZE

Test: builds
Bug: 79106666
Change-Id: I26aba694fd3d6a2feed7601524dde52569ba41e2
diff --git a/core/product.mk b/core/product.mk
index 55e1d04..76ca140 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -395,6 +395,7 @@
 _product_stash_var_list += \
 	BOARD_SYSTEMIMAGE_PARTITION_RESERVED_SIZE \
 	BOARD_VENDORIMAGE_PARTITION_RESERVED_SIZE \
+	BOARD_PRODUCTIMAGE_PARTITION_RESERVED_SIZE \
 	BOARD_SUPER_PARTITION_SIZE \
 	BOARD_SUPER_PARTITION_PARTITION_LIST \