The kernel build / configuration is now unnecessarily complicated:

• There is a generic.nix and the oddly named manual-config.nix.
• The kernel config can be specified in multiple ways, namely as a string and as an attribute set. The latter doesn't appear to be used anywhere. Kernel configuration can also come from pkgs/top-level/platforms.nix.
• manual-config.nix has an "import from derivation" feature, which appears to be unused except by the NixOS system.requiredKernelConfig option, which is itself unused (AFAIK).

We should merge generic.nix and manual-config.nix, drop the configuration via attribute sets and platforms.nix (alternatively, convert the string configuration to attribute set), and drop the "import from derivation" stuff.