kernel: I hope to fix the dtbs thing

I thought $arch was defined. Now I'm using karch, that should serve for cross
building as well.

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

@@ -122,7 +122,7 @@ let
         mkdir -p $out/lib/firmware
       '') + (if (platform ? kernelDTB && platform.kernelDTB) then ''
  	make $makeFlags "''${makeFlagsArray[@]}" dtbs
-        cp arch/$arch/boot/dts/*dtb $out
+        cp $buildRoot/arch/$karch/boot/dts/*dtb $out
       '' else "") + (if isModular then ''
         make modules_install $makeFlags "''${makeFlagsArray[@]}" \
           $installFlags "''${installFlagsArray[@]}"
@@ -225,6 +225,8 @@ stdenv.mkDerivation ((drvAttrs config stdenv.platform (kernelPatches ++ nativeKe
     "ARCH=${stdenv.platform.kernelArch}"
   ];
 
+  karch = stdenv.platform.kernelArch;
+
   crossAttrs = let cp = stdenv.cross.platform; in
     (drvAttrs crossConfig cp (kernelPatches ++ crossKernelPatches) crossConfigfile) // {
       makeFlags = commonMakeFlags ++ [
@@ -232,6 +234,8 @@ stdenv.mkDerivation ((drvAttrs config stdenv.platform (kernelPatches ++ nativeKe
         "CROSS_COMPILE=$(crossConfig)-"
       ];
 
+      karch = cp.kernelArch;
+
       # !!! uboot has messed up cross-compiling, nativeDrv builds arm tools on x86,
       # crossDrv builds x86 tools on x86 (but arm uboot). If this is fixed, uboot
       # can just go into buildInputs (but not nativeBuildInputs since cp.uboot