Revert "linuxManualConfig: unpack directly into $dev"

This reverts commit 7de3f08ce3.

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

@@ -116,20 +116,6 @@ let
             hash = "sha256-bBOyJcP6jUvozFJU0SPTOf3cmnTQ6ZZ4PlHjiniHXLU=";
           });
 
-      preUnpack = ''
-        # The same preUnpack is used to build the configfile,
-        # which does not have $dev.
-        if [ -n "$dev" ]; then
-            mkdir -p $dev/lib/modules/${modDirVersion}
-            cd $dev/lib/modules/${modDirVersion}
-        fi
-      '';
-
-      postUnpack = ''
-        mv -Tv "$sourceRoot" source 2>/dev/null || :
-        export sourceRoot=$PWD/source
-      '';
-
       postPatch = ''
         sed -i Makefile -e 's|= depmod|= ${buildPackages.kmod}/bin/depmod|'
 
@@ -200,12 +186,6 @@ let
 
       buildFlags = [
         "KBUILD_BUILD_VERSION=1-NixOS"
-
-        # Set by default in the kernel since a73619a845d5,
-        # replicated here to apply to older versions.
-        # Makes __FILE__ relative to the build directory.
-        "KCPPFLAGS=-fmacro-prefix-map=$(sourceRoot)/="
-
         kernelConf.target
         "vmlinux"  # for "perf" and things like that
       ] ++ optional isModular "modules"
@@ -282,6 +262,7 @@ let
       ];
 
       postInstall = optionalString isModular ''
+        mkdir -p $dev
         cp vmlinux $dev/
         if [ -z "''${dontStrip-}" ]; then
           installFlagsArray+=("INSTALL_MOD_STRIP=1")
@@ -291,7 +272,12 @@ let
         unlink $out/lib/modules/${modDirVersion}/build
         unlink $out/lib/modules/${modDirVersion}/source
 
-        mkdir $dev/lib/modules/${modDirVersion}/build
+        mkdir -p $dev/lib/modules/${modDirVersion}/{build,source}
+
+        # To save space, exclude a bunch of unneeded stuff when copying.
+        (cd "$NIX_BUILD_TOP" && cd "$sourceRoot" &&
+         rsync --archive --prune-empty-dirs \
+            * $dev/lib/modules/${modDirVersion}/source/)
 
         cd $dev/lib/modules/${modDirVersion}/source