[build] Add system_image property to package()

Change-Id: I7fab62ab87b3cc8b13832c4126a5e31380b7e5c3
diff --git a/BUILD.gn b/BUILD.gn
index 789defe..9818f21 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -106,7 +106,10 @@
 }
 
 package("package") {
+  system_image = true
+
   package_name = "zlib"
+
   deps = [
     ":minizip",
     ":zlib",