kernel: Add a validity check for modDirVersion

Because if you get it wrong, you get a very confusing error message at
the end of the kernel build, which is quite painful as the build can
take a long time.

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

@@ -102,6 +102,13 @@ let
         make $makeFlags "''${makeFlagsArray[@]}" oldconfig
         runHook postConfigure
 
+        make $makeFlags prepare
+        actualModDirVersion="$(cat $buildRoot/include/config/kernel.release)"
+        if [ "$actualModDirVersion" != "${modDirVersion}" ]; then
+          echo "Error: modDirVersion specified in the Nix expression is wrong, it should be: $actualModDirVersion"
+          exit 1
+        fi
+
         # Note: we can get rid of this once http://permalink.gmane.org/gmane.linux.kbuild.devel/13800 is merged.
         buildFlagsArray+=("KBUILD_BUILD_TIMESTAMP=$(date -u -d @$SOURCE_DATE_EPOCH)")
       '';