Fix systemd failure with lock files

Systemd introduced a new policy described here:
http://lists.freedesktop.org/archives/systemd-devel/2011-March/001823.html



Add the "lock" group to buildroot to allow systemd to set this in place
at boot time

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>