[images] only build netboot when requested

When the netboot.sh script was introduced as a build contract, the optional
building of netboot images regressed. It now returns.

Change-Id: I52208a0de7c8aba199420eeb63a7ceb26c55709a
diff --git a/images/BUILD.gn b/images/BUILD.gn
index be0330c..8865cad 100644
--- a/images/BUILD.gn
+++ b/images/BUILD.gn
@@ -1030,7 +1030,6 @@
   testonly = true
   deps = [
     ":ids.txt",
-    ":netboot-script",
     ":paver-script",
     "zedboot",
   ]
diff --git a/packages/netboot b/packages/netboot
index 42bd4a9..6f19a7e 100644
--- a/packages/netboot
+++ b/packages/netboot
@@ -1,5 +1,6 @@
 {
     "labels": [
-        "//build/images:netboot"
+        "//build/images:netboot",
+        "//build/images:netboot-script"
     ]
 }