systemd.bbclass: add ${sysconfdir}/systemd/user to search path

systemd.bbclass: add ${sysconfdir}/systemd/user to search path

We already search for system units ${sysconfdir}/systemd/system but we
don't search for user units in corresponding directory under ${sysconfdir}.
Keep the behaviour consistent so that both unit types are searched in
${systemd_{system,user}_unitdir} and ${sysconfdir}/systemd/{system,user}.

Signed-off-by: Artur Kowalski <arturkow2000@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>