The main Makefile includes .config - on the other hand, it also
generates .config when using make *config. This leads to recursion
problems when we make the build depend on .config.

(.config is generated by *config. So after running make *config,
 make notices one of its includes has been updated -> it restarts
 to read the new contents. Unfortunately, the restart runs *config
 again, which updates .config again. You get the picture)

Therefore, we're aiming at a two phase system: If no .config exists,
allow only make *config and make clean/mrproper. Afterwards, allow
all targets. (It's not much different from what we have today)