Pack the update_engine_config.txt when using the A/B OTA updater.

This config file pulled directly from the source specifies the update
payload version supported by the code. When generating an incremental
update payload for the A/B updater, the generated payload format will
match the version supported by the old version.

Bug: 24406755
Test: `make dist` on Brillo.

Change-Id: Ib7ac6828d0d34eb31e786dbaae5d59ecf26c3576
diff --git a/core/Makefile b/core/Makefile
index ed69855..e86831b 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1565,8 +1565,12 @@
 	$(hide) PATH=$(foreach p,$(INTERNAL_USERIMAGES_BINARY_PATHS),$(p):)$$PATH MKBOOTIMG=$(MKBOOTIMG) \
 	    ./build/tools/releasetools/make_recovery_patch $(zip_root) $(zip_root)
 endif
+ifeq ($(AB_OTA_UPDATER),true)
+	@# When using the A/B updater, include the updater config files in the zip.
+	$(hide) $(ACP) $(TOPDIR)system/update_engine/update_engine.conf $(zip_root)/META/update_engine_config.txt
+endif
 ifeq ($(BREAKPAD_GENERATE_SYMBOLS),true)
-	# If breakpad symbols have been generated, add them to the zip.
+	@# If breakpad symbols have been generated, add them to the zip.
 	$(hide) $(ACP) -r $(TARGET_OUT_BREAKPAD) $(zip_root)/BREAKPAD
 endif
 	@# Zip everything up, preserving symlinks