put extra recovery keys in the product definition

Change-Id: I71637cd578b24891c10171aa59406848649be703
diff --git a/core/product.mk b/core/product.mk
index 249582b..c2b398e 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -71,6 +71,7 @@
     PRODUCT_CHARACTERISTICS \
     PRODUCT_COPY_FILES \
     PRODUCT_OTA_PUBLIC_KEYS \
+    PRODUCT_EXTRA_RECOVERY_KEYS \
     PRODUCT_PACKAGE_OVERLAYS \
     DEVICE_PACKAGE_OVERLAYS \
     PRODUCT_CONTRIBUTORS_FILE \