linux: ability to merge structured configs

This should make the composability of kernel configurations more straigthforward.

- now distinguish freeform options from tristate ones
- will look for a structured config in kernelPatches too
one can now access the structuredConfig from a kernel via linux_test.configfile.structuredConfig
in order to reinject it into another kernel, no need to rewrite the config from scratch

The following merge strategies are used in case of conflict:
-- freeform items must be equal or they conflict (mergeEqualOption)
-- for tristate (y/m/n) entries, I use the mergeAnswer strategy which takes the best available value, "best" being defined by the user (by default "y" > "m" > "n", e.g. if one entry is both marked "y" and "n", "y" wins)
-- if one item is both marked optional/mandatory, mandatory wins (mergeFalseByDefault)

pkgs/os-specific/linux/kernel/generic.nix

@@ -47,7 +47,6 @@
 , preferBuiltin ? stdenv.hostPlatform.platform.kernelPreferBuiltin or false
 , kernelArch ? stdenv.hostPlatform.platform.kernelArch
 
-, mkValueOverride ? null
 , ...
 }:
 
@@ -68,20 +67,26 @@ let
     ia32Emulation = true;
   } // features) kernelPatches;
 
-  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 (stdenv.hostPlatform.platform ? kernelExtraConfig) stdenv.hostPlatform.platform.kernelExtraConfig;
+  commonStructuredConfig = import ./common-config.nix {
+    inherit stdenv version ;
 
     features = kernelFeatures; # Ensure we know of all extra patches, etc.
   };
 
-  kernelConfigFun = baseConfig:
+  # extra config in legacy string format
+  extraConfig = extraConfig + lib.optionalString (stdenv.hostPlatform.platform ? kernelExtraConfig) stdenv.hostPlatform.platform.kernelExtraConfig;
+
+  intermediateNixConfig = configfile.moduleStructuredConfig.intermediateNixConfig;
+
+  structuredConfigFromPatches =
+        map ({extraStructuredConfig ? {}, ...}: {settings=extraStructuredConfig;}) kernelPatches;
+
+  # appends kernel patches extraConfig
+  kernelConfigFun = baseConfigStr:
     let
       configFromPatches =
         map ({extraConfig ? "", ...}: extraConfig) kernelPatches;
-    in lib.concatStringsSep "\n" ([baseConfig] ++ configFromPatches);
+    in lib.concatStringsSep "\n" ([baseConfigStr] ++ configFromPatches);
 
   configfile = stdenv.mkDerivation {
     inherit ignoreConfigErrors autoModules preferBuiltin kernelArch;
@@ -131,7 +136,30 @@ let
     installPhase = "mv $buildRoot/.config $out";
 
     enableParallelBuilding = true;
-  };
+
+    passthru = rec {
+
+      module = import ../../../../nixos/modules/system/boot/kernel_config.nix;
+      # used also in apache
+      # { modules = [ { options = res.options; config = svc.config or svc; } ];
+      #   check = false;
+      # The result is a set of two attributes
+      moduleStructuredConfig = (lib.evalModules {
+        modules = [
+          module
+          { settings = commonStructuredConfig; }
+          { settings = structuredExtraConfig; }
+        ]
+        ++  structuredConfigFromPatches
+        ;
+      }).config;
+
+      #
+      structuredConfig = moduleStructuredConfig.settings;
+    };
+
+
+  }; # end of configfile derivation
 
   kernel = (callPackage ./manual-config.nix {}) {
     inherit version modDirVersion src kernelPatches stdenv extraMeta configfile;
@@ -141,6 +169,7 @@ let
 
   passthru = {
     features = kernelFeatures;
+    inherit commonStructuredConfig;
     passthru = kernel.passthru // (removeAttrs passthru [ "passthru" ]);
   };