fix(sys-apps/dbus) machine-id fixes

Properly create the /var/lib/dbus/machine-id symlink.
Create a "known" machine id as part of the build process, this will be
overwritten by the boot sequence with a "unique" one when booting for
the first time.
Merge pull request #421 from gregkh/dbus

fix(sys-apps/dbus) machine-id fixes