Add a machineid module.

The module creates /etc/machine-id and/or /var/lib/dbus/machine-id. By
default, it is enabled and creates both files, making the latter a
symlink to the former.
machineid: Add missing import.
machineid: Add modules.desc.
Merge pull request #171 from calamares/machineid-module

Add a machineid module.