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.

Only compile tested, might not work.

Fixes #65

Hopefully fixes #68

These models have only 4MB flash so Gluon won't fit on them with all
these modules.

Fixes #60

Closes #63