Add a rule for brillo_update_payload. With the changes to build/core/Makefile, the script will be installed to $(HOST_OUT_EXECUTABLES) to generate the payload for A/B update. Bug: 25715402 Test: `mmma system/update_engine` on an Android target on Linux. Change-Id: Ia3fadae1d4c4b7ba65dc40f68440728e50a2d984