[build] Add system_image property to package()

Change-Id: Ibe225906faf4d5a00f6ca58df41a76e3d121400a
diff --git a/BUILD.gn b/BUILD.gn
index 5b9ed24..d6114a4 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -171,6 +171,8 @@
 
 
 package("package") {
+  system_image = true
+
   package_name = "libxml2"
 
   deps = [