Fix ManifestParseError when first child node is comment

If the first line of manifest.xml is a XML comment, root.childNodes[0]
is not a 'manifest' element node. The python minidom module will makes
a 'Comment' node as root.childNodes[0]. Since the original code only
checks whether the first child node is 'manifest', it couldn't do any
command including 'sync' due to the 'ManifestParseError' exception. This
patch could allow the comments between '<?xml ...?>' and '<manifest>' in
the manifest.xml file.

Change-Id: I0b81dea4f806965eca90f704c8aa7df49c579402

manifest_xml.py

@@ -309,12 +309,14 @@ class XmlManifest(object):
     if not root or not root.childNodes:
       raise ManifestParseError("no root node in %s" % (path,))
 
-    config = root.childNodes[0]
-    if config.nodeName != 'manifest':
+    for manifest in root.childNodes:
+      if manifest.nodeName == 'manifest':
+        break
+    else:
       raise ManifestParseError("no <manifest> in %s" % (path,))
 
     nodes = []
-    for node in config.childNodes:
+    for node in manifest.childNodes:
         if node.nodeName == 'include':
             name = self._reqatt(node, 'name')
             fp = os.path.join(include_root, name)