[build] Add system_image property to package()

Change-Id: I2bd3689c6037b602da33785fcef9be22e216aafe
diff --git a/BUILD.gn b/BUILD.gn
index 1235fee..ffedb30 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -30,6 +30,8 @@
 import("//packages/package.gni")
 
 package("iperf") {
+  system_image = true
+
   deps = [
     ":iperf3",
   ]