Print deprecation warning when local_manifest.xml is used

The preferred way to specify local manifests is to drop the file(s)
in the local_manifests folder.  Print a deprecation warning when
the legacy local_manifest.xml file is used.

Change-Id: Ice85bd06fb612d6fcceeaa0755efd130556c4464

docs/manifest-format.txt

@@ -250,4 +250,6 @@ invocation, instructing repo to automatically download and manage
 these extra projects.
 
 Additional remotes and projects may also be added through a local
-manifest, stored in `$TOP_DIR/.repo/local_manifest.xml`.
+manifest, stored in `$TOP_DIR/.repo/local_manifest.xml`. This method
+is deprecated in favor of using multiple manifest files as mentioned
+above.

manifest_xml.py

@@ -300,6 +300,8 @@ class XmlManifest(object):
 
       local = os.path.join(self.repodir, LOCAL_MANIFEST_NAME)
       if os.path.exists(local):
+        print >>sys.stderr, 'warning: %s is deprecated; put local manifests in %s instead' % \
+                            (LOCAL_MANIFEST_NAME, LOCAL_MANIFESTS_DIR_NAME)
         nodes.append(self._ParseManifestXml(local, self.repodir))
 
       local_dir = os.path.abspath(os.path.join(self.repodir, LOCAL_MANIFESTS_DIR_NAME))