package/systemd: add missing dependencies

Systemd depends on mount and nologin from util-linux. Mount from busybox
is not fine, because its behavior differs from util-linux mount. Busybox
mount does not handle correctly 'mount / -o remount' (should apply
options from /etc/fstab). Nologin should be present, because
systemd-sysusers uses it as shell for newly created system users.

Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>