Deprecate serve_only for archive_dir.

Also change when the devserver will attempt to generate a payload, and
when it will attempt to generate a payload from the latest local build.

Previously, generation was attempted when no --image or --payload was
specified, and if archive_dir's serve_only mode was off.

Since serve_only is gone, we now just check that we're within chroot
before attempting to generate a payload.

Before attempting to generate a payload from the most recent local
build, we check that the directory specified by the label (if provided)
doesn't contain an update file.

BUG=chromium:261775
TEST=unittests updated
cbuildbot --remote mario-paladin

Change-Id: If094db197005836d9fcad5fb63ffcb8470be3706
Reviewed-on: https://gerrit.chromium.org/gerrit/63363
Reviewed-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: Joy Chen <joychen@chromium.org>
Tested-by: Joy Chen <joychen@chromium.org>
Commit-Queue: Joy Chen <joychen@chromium.org>
diff --git a/common_util_unittest.py b/common_util_unittest.py
index ac8a273..34fea31 100755
--- a/common_util_unittest.py
+++ b/common_util_unittest.py
@@ -47,8 +47,8 @@
           build_path, build_artifact.STATEFUL_UPDATE_FILE), 'w') as f:
           f.write('STATEFUL_UPDATE_FILE')
         with open(os.path.join(
-          build_path, devserver_constants.ROOT_UPDATE_FILE), 'w') as f:
-          f.write('ROOT_UPDATE_FILE')
+          build_path, devserver_constants.UPDATE_FILE), 'w') as f:
+          f.write('UPDATE_FILE')
 
   def tearDown(self):
     shutil.rmtree(self._static_dir)