Make -notdefault a default manifest group

When trying to render manifest with SHAs, projects in group notdefault
caused the following crash:

    Traceback (most recent call last):
      File ".repo/repo/main.py", line 385, in <module>
        _Main(sys.argv[1:])
      File ".repo/repo/main.py", line 365, in _Main
        result = repo._Run(argv) or 0
      File ".repo/repo/main.py", line 137, in _Run
        result = cmd.Execute(copts, cargs)
      File ".repo/repo/subcmds/manifest.py", line 129, in Execute
        self._Output(opt, manifest)
      File ".repo/repo/subcmds/manifest.py", line 79, in _Output
        peg_rev = opt.peg_rev)
      File ".repo/repo/manifest_xml.py", line 199, in Save
        p.work_git.rev_parse(HEAD + '^0'))
      File ".repo/repo/project.py", line 2035, in runner
        capture_stderr = True)
      File ".repo/repo/git_command.py", line 215, in __init__
        raise GitError('%s: %s' % (command[1], e))
    error.GitError: rev-parse: [Errno 2] No such file or directory: 'prebuilts/eclipse-build-deps'

This patch resolves the issue by making sure that -notdefault is always
used as a default manifest group so that notdefault projects are not
rendered out by the manifest subcmd.

Change-Id: I4a8bd18ea7600309f39ceff1b1ab6e1ff3adf21d
Signed-off-by: Matt Gumbel <matthew.k.gumbel@intel.com>
diff --git a/manifest_xml.py b/manifest_xml.py
index 36f8ef8..73f6c42 100644
--- a/manifest_xml.py
+++ b/manifest_xml.py
@@ -139,9 +139,8 @@
     mp = self.manifestProject
 
     groups = mp.config.GetString('manifest.groups')
-    if not groups:
-      groups = 'all'
-    groups = [x for x in re.split(r'[,\s]+', groups) if x]
+    if groups:
+      groups = [x for x in re.split(r'[,\s]+', groups) if x]
 
     doc = xml.dom.minidom.Document()
     root = doc.createElement('manifest')