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

Otherwise, `make cleanall` fails...

Explicitly re-build prepare-tmpinfo after the feeds have been installed, as it
isn't done automatically anymore since we have the gluon-tools target.

Also, more cleanup.

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