Pack META/ab_partitions.txt when using the A/B OTA updater.

The list of A/B partitions updated is product-specific. Normally you
would update boot and system, but the product could require to update
other partitions during a system update. This patch packages the list
of A/B partitions configured to be updated when using the A/B updater.
The list can be set in the product makefile by setting the make
variable UPDATE_AB_PARTITIONS.

Bug: 24387863
Test: `make dist` creates the META/ab_partitions.txt in the target_files.zip

Change-Id: I4226ab05c7c7cd74721b06f1da64c6cf4d0e52ac
diff --git a/core/Makefile b/core/Makefile
index e86831b..7f7aca4 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1568,6 +1568,9 @@
 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
+	$(hide) for part in $(UPDATE_AB_PARTITIONS); do \
+	  echo "$${part}" >> $(zip_root)/META/ab_partitions.txt; \
+	done
 endif
 ifeq ($(BREAKPAD_GENERATE_SYMBOLS),true)
 	@# If breakpad symbols have been generated, add them to the zip.