[build] Add system_image property to package()

Change-Id: Idfeda124a81b76f2437d1569a9adc491eb0598f7
diff --git a/BUILD.gn b/BUILD.gn
index f2fca44..f59e84e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -347,6 +347,8 @@
 
 package("sbase") {
   testonly = true
+  system_image = true
+
   deps = [
     ":all",
   ]