linux: translate config to structured config

Instead of using a string to describe kernel config, use a nix
attribute set, then converted to a string.
- allows to override the config, aka convert 'yes' into 'modules' or
vice-versa
- while for now merging different configs is still crude (last spec wins),
at least there should be only one CONFIG_XYZ value compared to the current string
config where the first defined would be used and others ignored.

[initial idea by copumpkin in 2016, a major rebase to 2018 by teto]

pkgs/os-specific/linux/kernel/manual-config.nix

@@ -24,7 +24,7 @@ in {
   modDirVersion ? version,
   # The kernel source (tarball, git checkout, etc.)
   src,
-  # Any patches
+  # a list of { name=..., patch=..., extraConfig=...} patches
   kernelPatches ? [],
   # The kernel .config file
   configfile,