Avoid calling MergeUpdateEngineConfig for non-ab target

super image generation is failing for non-ab target as it is not able to find update_engine_config.txt while calling MergeUpdateEngineConfig.

META/update_engine_config.txt exists when AB_OTA_UPDATER is true.
Avoid calling MergeUpdateEngineConfig for non-ab target.

Bug: 290811619
Change-Id: Ie88b31adce15df87f73031e1f5bff0d825601e0c
diff --git a/tools/releasetools/merge/merge_meta.py b/tools/releasetools/merge/merge_meta.py
index b61f039..81c3510 100644
--- a/tools/releasetools/merge/merge_meta.py
+++ b/tools/releasetools/merge/merge_meta.py
@@ -124,10 +124,10 @@
         merged_meta_dir=merged_meta_dir,
         file_name=file_name)
 
-  MergeUpdateEngineConfig(
-      framework_meta_dir,
-      vendor_meta_dir, merged_meta_dir,
-  )
+  if OPTIONS.merged_misc_info.get('ab_update') == 'true':
+    MergeUpdateEngineConfig(
+        framework_meta_dir,
+        vendor_meta_dir, merged_meta_dir)
 
   # Write the now-finalized OPTIONS.merged_misc_info.
   merge_utils.WriteSortedData(