[WSAP00026581] output lfs-fetch property in manifest file

[Description]
With this modification, users could generate lfs-supported manifest file with commands like "mtk_repo manifest -r -o my_manifest.xml"

CR-Id: WSAP00026581
Change-Id: Ic84719c4b71ff9ab56756a17ae362f4a22d99dde
diff --git a/manifest_xml.py b/manifest_xml.py
index afd1f39..41d92e9 100644
--- a/manifest_xml.py
+++ b/manifest_xml.py
@@ -306,6 +306,9 @@
       if p.sync_s:
         e.setAttribute('sync-s', 'true')
 
+      if p.lfs_fetch: 
+        e.setAttribute('lfs-fetch', 'true')
+
       if p.clone_depth:
         e.setAttribute('clone-depth', str(p.clone_depth))