systemctl-native: Rewrite in Python supporting preset-all and mask

systemctl-native: Rewrite in Python supporting preset-all and mask

Rewrite systemctl-native in Python so that extending/testing it is
easier.

Now that the systemd class sets up service presets instead of actively
enabling services, the 'enable' and 'disable' subcommands for systemctl
are not actually used anywhere.  As such, we can remove these to make
sure that nobody inadvertently introduces new uses of them.

This implementation covers `preset-all` and `mask` which are the only
options used in the current code, but should be readily extensible to
other commands.

We use `preset-all` at image construction time to populate the symlinks
used by systemd.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>