init: add --submodules to sync manifest submodules

repo sync can sync submodules via the --fetch-submodules option.
However, if the manifest repo has submodules, those will not be synced.
Having submodules in the manifest repo -- while not commonly done -- can
be useful for inheriting a manifest from another project using <include>
and layering changes on top of it.  In this way, you can avoid having to
deal with merge conflicts between your own manifests and the other
project's manifests (for example, if you're managing an Android fork).

Add a --submodule option to init that automatically syncs the submodules
in the manifest repo whenever the manifest repo changes.

Change-Id: I45d34f04517774c1462d7f233f482d1d81a332a8
Signed-off-by: Martin Kelly <mkelly@xevo.com>
diff --git a/manifest_xml.py b/manifest_xml.py
index 0859e1f..73e3496 100644
--- a/manifest_xml.py
+++ b/manifest_xml.py
@@ -393,6 +393,10 @@
   def IsArchive(self):
     return self.manifestProject.config.GetBoolean('repo.archive')
 
+  @property
+  def HasSubmodules(self):
+    return self.manifestProject.config.GetBoolean('repo.submodules')
+
   def _Unload(self):
     self._loaded = False
     self._projects = {}