store releasetools.py in the target_files zip

Save the device-specific releasetools.py module in the target_files
zip so that the OTA tools can use it without needing to get it from a
client.

Change-Id: I2dd5f4628290a8f2f56bd519b07d4ca8d92fddcd
diff --git a/core/Makefile b/core/Makefile
index 298eef1..d7602a5 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1243,6 +1243,7 @@
 	@# build them.
 	$(hide) mkdir -p $(zip_root)/META
 	$(hide) $(ACP) $(APKCERTS_FILE) $(zip_root)/META/apkcerts.txt
+	$(hide) if test -e $(tool_extensions)/releasetools.py; then $(ACP) $(tool_extensions)/releasetools.py $(zip_root)/META/; fi
 	$(hide)	echo "$(PRODUCT_OTA_PUBLIC_KEYS)" > $(zip_root)/META/otakeys.txt
 	$(hide) echo "recovery_api_version=$(PRIVATE_RECOVERY_API_VERSION)" > $(zip_root)/META/misc_info.txt
 	$(hide) echo "fstab_version=$(PRIVATE_RECOVERY_FSTAB_VERSION)" >> $(zip_root)/META/misc_info.txt