linuxManualConfig: unpack directly into $dev

linux is unusual in that we include its sources in an output.  There's
no point unpacking into /build when we're going to copy the sources
into $dev later.  Let's unpack directly into the final destination of
the code, and save copying a whole kernel source tree (often across
filesystems!).

This also means that Kbuild knows the location of the sources, which
will allow us to install the GDB scripts — some scripts are generated,
and some are not, so the generated ones end up in the build directory,
accompanied by symlinks to the non-generated ones in the source
directory.

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

@@ -115,6 +115,20 @@ 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|'
 
@@ -185,6 +199,12 @@ 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"
@@ -261,7 +281,6 @@ let
       ];
 
       postInstall = optionalString isModular ''
-        mkdir -p $dev
         cp vmlinux $dev/
         if [ -z "''${dontStrip-}" ]; then
           installFlagsArray+=("INSTALL_MOD_STRIP=1")
@@ -271,12 +290,7 @@ let
         unlink $out/lib/modules/${modDirVersion}/build
         unlink $out/lib/modules/${modDirVersion}/source
 
-        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/)
+        mkdir $dev/lib/modules/${modDirVersion}/build
 
         cd $dev/lib/modules/${modDirVersion}/source