Add build task for cts_root.

This adds the configuration necessary to allow the cts_root
test suite to be built.

Bug: 162933947
Test: m cts_root

Change-Id: I4732bf82d3cedc77e30d7fa5eff285d382992853
diff --git a/core/main.mk b/core/main.mk
index 14ead7c..96cc31f 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -85,6 +85,8 @@
 -include test/app_compat/csuite/tools/build/config.mk
 # CATBox-specific config.
 -include test/catbox/tools/build/config.mk
+# CTS-Root-specific config.
+-include test/cts-root/tools/build/config.mk
 
 # Clean rules
 .PHONY: clean-dex-files