[build] Add system_image property to package()

Change-Id: If5ea83ea89d4445f1f092470be88cccf807bdada
diff --git a/BUILD.gn b/BUILD.gn
index 55df11c..9f3c194 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -29,6 +29,8 @@
 import("//packages/package.gni")
 
 package("git") {
+  system_image = true
+
   deps = [
     ":all",
   ]