Perform a lazy umount.

The umount was failing on some systems. Performing a -l seems
to fix the issue.

BUG=None
TEST=manual run

Change-Id: I66dd1ca987de6769c81c88bc6c936a26d72a6bb5
Reviewed-on: https://gerrit.chromium.org/gerrit/36062
Tested-by: Yusuf Mohsinally <mohsinally@chromium.org>
Reviewed-by: Gediminas Ramanauskas <gedis@chromium.org>
Tested-by: Gediminas Ramanauskas <gedis@chromium.org>
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
diff --git a/server/cros/faftsequence.py b/server/cros/faftsequence.py
index 9460520..78b7026 100644
--- a/server/cros/faftsequence.py
+++ b/server/cros/faftsequence.py
@@ -318,7 +318,7 @@
         code = utils.system(
                'grep -qE "(Test Build|testimage-channel)" %s/etc/lsb-release' %
                tmp_dir, ignore_status=True)
-        utils.system('sudo umount %s' % tmp_dir)
+        utils.system('sudo umount -l %s' % tmp_dir)
         os.removedirs(tmp_dir)
         if code != 0:
             raise error.TestError(