[build] Add system_image property to package()

Change-Id: Ia8988816fe496d9081580741cf5b1195fd9ad509
diff --git a/BUILD.gn b/BUILD.gn
index af7292d..ef208e3 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -29,6 +29,8 @@
 import("//packages/package.gni")
 
 package("curl") {
+  system_image = true
+
   deps = [
     ":curl_bin",
     ":libcurl",