Add ro.build.ab_update property.

Set ro.build.ab_update if AB_OTA_UPDATER is true. Device will depend on
this property to call the proper update API.

Change-Id: I5916ef5b30ea3351d323382f7ce83a17df2b0a70
diff --git a/core/Makefile b/core/Makefile
index d4dc002..37c11b6 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -204,6 +204,7 @@
 			BUILD_DISPLAY_ID="$(BUILD_DISPLAY_ID)" \
 			BUILD_NUMBER="$(BUILD_NUMBER)" \
 			BOARD_BUILD_SYSTEM_ROOT_IMAGE="$(BOARD_BUILD_SYSTEM_ROOT_IMAGE)" \
+			AB_OTA_UPDATER="$(AB_OTA_UPDATER)" \
 			PLATFORM_VERSION="$(PLATFORM_VERSION)" \
 			PLATFORM_SECURITY_PATCH="$(PLATFORM_SECURITY_PATCH)" \
 			PLATFORM_BASE_OS="$(PLATFORM_BASE_OS)" \