Instead of manually adding which 'features' a kernel has based on its config here, just generate a nixexpr representation of the whole config and let places that need it (e.g. NixOS modules) decide how to map config options to features.
Note that if your kernel requires firmware whose hex file sits directly in ${src}/firmware rather than in a subdirectory thereof, you may need this patch for now: https://lkml.org/lkml/2012/7/31/724
The goal of this function is to make it possible to build a kernel with a user provided .config.
As a secondary goal, it will extract NixOS-relevant features from the config automatically.
As a tertiary goal, the build will aim to be simpler than the current generic kernel builder.sh.
Unfortunately, that simplicity is offset by the complexity of the feature extraction, especially since nix segfaults when trying to split the file into lines (so an import from a derivation is used)
