Add systemd support for at.
This patch mainly comes from meta-systemd.
[YOCTO #4420]
(From OE-Core rev: b71670bfbd8e394521cfd969eeafad4adc34c285)
Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>