Revert "Disable building of TLK, to unbreak master build."

This reverts commit 517e371d0ce4fb5ea46dfe845f44ccae69aed1a0.

Test: build successful
Bug: 31609346
Change-Id: Ie5e617f9498655d6bed0426c435191f6a92693ff
diff --git a/device.mk b/device.mk
index 703b869..40e0d43 100644
--- a/device.mk
+++ b/device.mk
@@ -26,7 +26,7 @@
 # prebuilts.  To force using the prebuilt while having the source, set:
 # SECURE_OS_BUILD=false
 ifeq ($(wildcard vendor/nvidia/dragon-tlk/tlk),vendor/nvidia/dragon-tlk/tlk)
-    SECURE_OS_BUILD ?= client_only
+    SECURE_OS_BUILD ?= tlk
 endif
 
 ifeq ($(TARGET_PREBUILT_KERNEL),)