feat(set_lsb_release): Add special "sdk" track without an update server.

I want to start including version info in SDK builds as an alternative
scheme to the existing "chroot_version_hooks" system which always
assumes freshly unpacked SDKs are the latest regardless of what version
they actually were.

build_library/set_lsb_release

@@ -36,7 +36,11 @@ if [[ -n "${FLAGS_production_track}" ]]; then
   COREOS_VERSION_TRACK="${FLAGS_production_track}"
   COREOS_VERSION_DESCRIPTION="${COREOS_VERSION_STRING} (Official Build) \
 ${COREOS_VERSION_TRACK} $FLAGS_board test"
+  if [[ "${FLAGS_production_track}" != "sdk" ]]; then
     COREOS_VERSION_AUSERVER="https://api.core-os.net/v1/update/"
+  else
+    COREOS_VERSION_AUSERVER=""
+  fi
   COREOS_VERSION_DEVSERVER=""
 else
   # Developer hand-builds