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/tools/buildinfo.sh b/tools/buildinfo.sh
index 1329be7..89af758 100755
--- a/tools/buildinfo.sh
+++ b/tools/buildinfo.sh
@@ -23,6 +23,9 @@
 if [ -n "$BOARD_BUILD_SYSTEM_ROOT_IMAGE" ] ; then
   echo "ro.build.system_root_image=$BOARD_BUILD_SYSTEM_ROOT_IMAGE"
 fi
+if [ -n "$AB_OTA_UPDATER" ] ; then
+  echo "ro.build.ab_update=$AB_OTA_UPDATER"
+fi
 echo "ro.product.model=$PRODUCT_MODEL"
 echo "ro.product.brand=$PRODUCT_BRAND"
 echo "ro.product.name=$PRODUCT_NAME"