Build and use ota_from_target_files as a module.

Bug: 63866463
Test: TreeHugger
Test: m -j otapackage
Test: `m -j otatools-package`. Check bin/ota_from_target_files is
      available. Build an OTA package with standalone otatools.
Change-Id: I533dee1c6997d233601e7f4eae637ff5743ed637
diff --git a/core/config.mk b/core/config.mk
index ee87632..db4edcb 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -608,6 +608,7 @@
 LPMAKE := $(HOST_OUT_EXECUTABLES)/lpmake$(HOST_EXECUTABLE_SUFFIX)
 BUILD_IMAGE := $(HOST_OUT_EXECUTABLES)/build_image$(HOST_EXECUTABLE_SUFFIX)
 BUILD_SUPER_IMAGE := $(HOST_OUT_EXECUTABLES)/build_super_image$(HOST_EXECUTABLE_SUFFIX)
+OTA_FROM_TARGET_FILES := $(HOST_OUT_EXECUTABLES)/ota_from_target_files$(HOST_EXECUTABLE_SUFFIX)
 
 PROGUARD_HOME := external/proguard
 PROGUARD := $(PROGUARD_HOME)/bin/proguard.sh
@@ -623,7 +624,6 @@
 FUTILITY := $(HOST_OUT_EXECUTABLES)/futility-host
 VBOOT_SIGNER := $(HOST_OUT_EXECUTABLES)/vboot_signer
 FEC := $(HOST_OUT_EXECUTABLES)/fec
-BRILLO_UPDATE_PAYLOAD := $(HOST_OUT_EXECUTABLES)/brillo_update_payload
 
 DEXDUMP := $(HOST_OUT_EXECUTABLES)/dexdump$(BUILD_EXECUTABLE_SUFFIX)
 PROFMAN := $(HOST_OUT_EXECUTABLES)/profman