[build] Add system_image property to package()

Change-Id: I2058c0727436c4e6cf9d5f43c570df15f6169095
diff --git a/BUILD.gn b/BUILD.gn
index af4716f..1c71d43 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -5,6 +5,8 @@
 import("//packages/package.gni")
 
 package("openssh-portable") {
+  system_image = true
+
   deps = [
     ":all",
   ]
diff --git a/fuchsia/developer-keys/BUILD.gn b/fuchsia/developer-keys/BUILD.gn
index 4cdc4a6..494b619 100644
--- a/fuchsia/developer-keys/BUILD.gn
+++ b/fuchsia/developer-keys/BUILD.gn
@@ -12,6 +12,8 @@
 }
 
 package("developer-keys") {
+  system_image = true
+
   deps = [
     ":all",
   ]