[build] Add system_image property to package()

Change-Id: I0f30455261ba9c3c8f471789fbf613995ec28477
diff --git a/BUILD.gn b/BUILD.gn
index d24babf..9b511b6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -5,6 +5,8 @@
 import("//packages/package.gni")
 
 package("lua") {
+  system_image = true
+
   deps = [
     ":lua_bin",
   ]