Makefile/linux: make the features selectable

Right now, we always include all the config files for Linux when generating the .config. It would be very nice if we could select which config files we would like to use for Linux compilation. This would make it easy for people to compile their own kernel with the defined set of features they would like without the granularity of single config options.

Of course, let's not add too many of such "features", but we definitely could add an opt-in gpu support for example.

Fixes: #18 (closed)