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/generic.nix

@@ -15,9 +15,12 @@
 , # Allows overriding the default defconfig
   defconfig ? null
 
-, # Overrides to the kernel config.
+, # Legacy overrides to the intermediate kernel config, as string
   extraConfig ? ""
 
+, # kernel intermediate config overrides, as a set
+ structuredExtraConfig ? {}
+
 , # The version number used for the module directory
   modDirVersion ? version
 
@@ -42,6 +45,7 @@
 , preferBuiltin ? hostPlatform.platform.kernelPreferBuiltin or false
 , kernelArch ? hostPlatform.platform.kernelArch
 
+, mkValueOverride ? null
 , ...
 } @ args:
 
@@ -59,8 +63,9 @@ let
     netfilterRPFilter = true;
   } // features) kernelPatches;
 
-  config = import ./common-config.nix {
-    inherit stdenv version ;
+  intermediateNixConfig = import ./common-config.nix {
+    inherit stdenv version structuredExtraConfig mkValueOverride;
+
     # append extraConfig for backwards compatibility but also means the user can't override the kernelExtraConfig part
     extraConfig = extraConfig + lib.optionalString (hostPlatform.platform ? kernelExtraConfig) hostPlatform.platform.kernelExtraConfig;
 
@@ -79,7 +84,7 @@ let
 
     generateConfig = ./generate-config.pl;
 
-    kernelConfig = kernelConfigFun config;
+    kernelConfig = kernelConfigFun intermediateNixConfig;
     passAsFile = [ "kernelConfig" ];
 
     depsBuildBuild = [ buildPackages.stdenv.cc ];

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,