package/dbus: fix machine-id when using systemd

When systemd is used, /var/lib/dbus becomes dangling symlink, because
nobody does mkdir /tmp/dbus, so /var/lib/dbus/machine-id could not be
written. On SysVinit systems there is init script that creates
/tmp/dbus.

This patch preserves old behavior for SysVinit systems, and introduces
new one for systemd-booted systems: /var/lib/dbus is a persistent
directory, it holds symlink /var/lib/dbus/machine-id -> /etc/machine-id
as machine-id(5) suggests, and /etc/machine-id is managed by systemd.

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