kernel: Reduce peak disk usage during build

Since commit f620b1b693ec25af, the build directory is located inside the
source directory. Thus, the `cp -dpR` copies gigabytes worth of .o files
only to be deleted later on when we trim all non-essential files from
`$dev/lib/modules/${modDirVersion}/source/` thus causing a significant
amount of wasted I/O and peak disk usage.

As `cp` doesn't come with a `--exclude` flag, use rsync. And throw out
the Documentation folder while at it.

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

···
-{ buildPackages, runCommand, nettools, bc, bison, flex, perl, gmp, libmpc, mpfr, openssl
+{ buildPackages, runCommand, nettools, bc, bison, flex, perl, rsync, gmp, libmpc, mpfr, openssl
 , libelf
 , utillinux
 , writeTextFile
···
         unlink $out/lib/modules/${modDirVersion}/build
         unlink $out/lib/modules/${modDirVersion}/source
 
-        mkdir -p $dev/lib/modules/${modDirVersion}/build
-        cp -dpR .. $dev/lib/modules/${modDirVersion}/source
+        mkdir -p $dev/lib/modules/${modDirVersion}/{build,source}
+
+        # To save space, exclude a bunch of unneeded stuff when copying.
+        (cd .. && rsync --archive --prune-empty-dirs \
+            --exclude='/build/' \
+            --exclude='/Documentation/' \
+            * $dev/lib/modules/${modDirVersion}/source/)
+
         cd $dev/lib/modules/${modDirVersion}/source
 
         cp $buildRoot/{.config,Module.symvers} $dev/lib/modules/${modDirVersion}/build
···
   enableParallelBuilding = true;
 
   depsBuildBuild = [ buildPackages.stdenv.cc ];
-  nativeBuildInputs = [ perl bc nettools openssl gmp libmpc mpfr ]
+  nativeBuildInputs = [ perl bc nettools openssl rsync gmp libmpc mpfr ]
       ++ optional (stdenv.hostPlatform.platform.kernelTarget == "uImage") buildPackages.ubootTools
       ++ optional (stdenv.lib.versionAtLeast version "4.14") libelf
       ++ optional (stdenv.lib.versionAtLeast version "4.15") utillinux