Add the product version to target_files.zip.

When using the A/B updater, include the product version string to
target_files.zip.
This is needed by the Omaha backend.

BUG: 24743570
Change-Id: Idaba06624c1de452b1e1f9d3ebec851229af987a
diff --git a/core/Makefile b/core/Makefile
index b7a457d..b1b3d13 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1606,6 +1606,9 @@
 	$(hide) for part in $(UPDATE_AB_PARTITIONS); do \
 	  echo "$${part}" >> $(zip_root)/META/ab_partitions.txt; \
 	done
+ifdef OSRELEASED_DIRECTORY
+	$(hide) $(ACP) $(TARGET_OUT_ETC)/$(OSRELEASED_DIRECTORY)/product_version $(zip_root)/META/product_version.txt
+endif
 endif
 ifeq ($(BREAKPAD_GENERATE_SYMBOLS),true)
 	@# If breakpad symbols have been generated, add them to the zip.