Rename UPDATE_AB_PARTITIONS to AB_OTA_PARTITIONS.

In order to have all the A/B updater variables in the same AB_OTA_
namespace we rename UPDATE_AB_PARTITIONS to AB_OTA_PARTITIONS.

Bug: None
Test: `make dist` includes the partitions.txt file if properly configured.

Change-Id: Ied4faabd7e2f0ab8a5007afb44ad2f38df8d630d
diff --git a/core/Makefile b/core/Makefile
index 7e7dd77..055372f 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1608,7 +1608,7 @@
 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 \
+	$(hide) for part in $(AB_OTA_PARTITIONS); do \
 	  echo "$${part}" >> $(zip_root)/META/ab_partitions.txt; \
 	done
 ifdef OSRELEASED_DIRECTORY