Add first attempt at linux/kernel/manual-config.

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)

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

@@ -0,0 +1,81 @@
+{ stdenv, runCommand }:
+
+{ version, modDirVersion ? version, src, patches ? [], config }:
+
+with stdenv.lib;
+
+let
+
+  # Function to parse the config file to get the features supported
+  readFeatures = config:
+    let
+      # !!! Original causes recursion too deep, need to import from derivation
+      # linesWithComments = splitString "\n" (builtins.readFile config);
+      lines = import "${runCommand "lines.nix" {} ''
+        echo "[" >> $out
+        while read line; do
+            if [ -n "$line" ] && [ `expr index "$line" "#"` -ne 1 ]; then
+                echo "${"''"}" >> $out
+                echo $(echo $line | sed "s@${"''"}@\$\{\"${"''"}\"}@g")"${"''"}" >> $out
+            fi
+        done < ${config} 
+        echo "]" >> $out
+      ''}";
+
+      nvpairs = map (s: let split = splitString "=" s; fst = head split; in {
+        name = substring (stringLength "CONFIG_") (stringLength fst) fst;
+        value = head (tail split);
+      }) lines;
+
+      configAttrs = listToAttrs nvpairs;
+
+      getValue = option:
+        if hasAttr option configAttrs then getAttr option configAttrs else null;
+
+      isYes = option: (getValue option) == "y";
+    in
+
+    {
+      modular = isYes "MODULES";
+    };
+
+  features = readFeatures config;
+
+in
+
+stdenv.mkDerivation ({
+  name = "linux-${version}";
+
+  enableParallelBuilding = true;
+
+  passthru = {
+    inherit version modDirVersion features;
+  };
+
+  inherit patches src;
+
+  postPatch = ''
+    for mf in $(find -name Makefile); do
+        echo "stripping FHS paths in \`$mf'..."
+        sed -i "$mf" -e 's|/usr/bin/||g ; s|/bin/||g'
+    done
+  '';
+
+  configurePhase = ''
+    runHook preConfigure
+    ln -sv ${config} .config
+    make oldconfig
+    runHook postConfigure
+  '';
+
+  INSTALL_PATH = "$out";
+} // optionalAttrs features.modular {
+  MODLIB = "$out/lib/modules/${modDirVersion}";
+
+  INSTALL_MOD_STRIP = "1";
+
+  postInstall = ''
+    make modules_install $makeFlags "$\{makeFlagsArray[@]}" \
+      $installFlags "$\{installFlagsArray[@]}"
+  '';
+})

pkgs/top-level/all-packages.nix

@@ -5856,6 +5856,11 @@ let
   linux = linuxPackages.kernel;
   linuxPackages = linuxPackages_3_2;
 
+  # A function to build a manually-configured kernel
+  linuxManualConfig = import ../os-specific/linux/kernel/manual-config.nix {
+    inherit stdenv runCommand;
+  };
+
   keyutils = callPackage ../os-specific/linux/keyutils { };
 
   libselinux = callPackage ../os-specific/linux/libselinux { };