They aren't used by default and can simply be installed through opkg if needed.

This allows us to use separate .config files per target. Also, don't use
OpenWRT's feeds script anymore, but symlink the feeds ourselves.

Also, more cleanup.

Introduce a new value "opkg_repo" in the site configuration.
The value should be a single domain name,
e.g. "http://opkg.services.ffki/attitude_adjustment/12.09/%S/packages".

This *should* fix the build on non-Linux systems like MacOS X.

Closes #63