kernel: buildLinux replaces import ./generic.nix

- defined buildLinux as generic.nix instead of manual-config.nix. This
makes kernel derivations a tad more similar to your typical derivations.
- moved $buildRoot to within the source folder, this way it doesn't have to be created before the unpackPhase
and make it easier to work on kernel source without running the unpackPhase

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

@@ -1,8 +1,8 @@
 { buildPackages, runCommand, nettools, bc, perl, gmp, libmpc, mpfr, openssl
+, ncurses ? null
 , libelf
 , utillinux
 , writeTextFile, ubootTools
-, hostPlatform
 }:
 
 let
@@ -34,7 +34,9 @@ in {
   # Use defaultMeta // extraMeta
   extraMeta ? {},
   # Whether to utilize the controversial import-from-derivation feature to parse the config
-  allowImportFromDerivation ? false
+  allowImportFromDerivation ? false,
+
+  hostPlatform
 }:
 
 let
@@ -86,8 +88,6 @@ let
       inherit src;
 
       preUnpack = ''
-        mkdir build
-        export buildRoot="$(pwd)/build"
       '';
 
       patches = map (p: p.patch) kernelPatches;
@@ -102,7 +102,25 @@ let
 
       configurePhase = ''
         runHook preConfigure
+
+        mkdir build
+        export buildRoot="$(pwd)/build"
+
+        echo "manual-config configurePhase buildRoot=$buildRoot pwd=$PWD"
+
+        if [[ -z "$buildRoot" || ! -d "$buildRoot" ]]; then
+          echo "set $buildRoot to the build folder please"
+          exit 1
+        fi
+
+        if [ -f "$buildRoot/.config" ]; then
+          echo "Could not link $buildRoot/.config : file exists"
+          exit 1
+        fi
         ln -sv ${configfile} $buildRoot/.config
+
+        # reads the existing .config file and prompts the user for options in
+        # the current kernel source that are not found in the file.
         make $makeFlags "''${makeFlagsArray[@]}" oldconfig
         runHook postConfigure
 
@@ -115,6 +133,8 @@ let
 
         # Note: we can get rid of this once http://permalink.gmane.org/gmane.linux.kbuild.devel/13800 is merged.
         buildFlagsArray+=("KBUILD_BUILD_TIMESTAMP=$(date -u -d @$SOURCE_DATE_EPOCH)")
+
+        cd $buildRoot
       '';
 
       buildFlags = [
@@ -136,7 +156,7 @@ let
 
       postInstall = ''
         mkdir -p $dev
-        cp $buildRoot/vmlinux $dev/
+        cp vmlinux $dev/
       '' + (optionalString installsFirmware ''
         mkdir -p $out/lib/firmware
       '') + (if (platform ? kernelDTB && platform.kernelDTB) then ''
@@ -151,7 +171,7 @@ let
         unlink $out/lib/modules/${modDirVersion}/source
 
         mkdir -p $dev/lib/modules/${modDirVersion}/build
-        cp -dpR ../$sourceRoot $dev/lib/modules/${modDirVersion}/source
+        cp -dpR .. $dev/lib/modules/${modDirVersion}/source
         cd $dev/lib/modules/${modDirVersion}/source
 
         cp $buildRoot/{.config,Module.symvers} $dev/lib/modules/${modDirVersion}/build
@@ -170,7 +190,7 @@ let
         # from drivers/ in the future; it adds 50M to keep all of its
         # headers on 3.10 though.
 
-        chmod u+w -R ../source
+        chmod u+w -R ..
         arch=$(cd $dev/lib/modules/${modDirVersion}/build/arch; ls)
 
         # Remove unused arches