kheaders: use standard naming for the temporary directory

If the kheaders archive generation is interrupted then this directory
may be left on disk and not ignored by git.
By using the standard naming schema for temporary files and directories
the default .gitignore and "make clean" rules will apply.
Suggested-by: NNicolas Schier <nicolas@fjasle.eu>
Suggested-by: NMasahiro Yamada <masahiroy@kernel.org>
Signed-off-by: NThomas Weißschuh <linux@weissschuh.net>
Reviewed-by: NNicolas Schier <nicolas@fjasle.eu>
Tested-by: NNicolas Schier <nicolas@fjasle.eu>
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

kernel/gen_kheaders.sh

@@ -7,7 +7,7 @@ set -e
 sfile="$(readlink -f "$0")"
 outdir="$(pwd)"
 tarfile=$1
-cpio_dir=$outdir/$tarfile.tmp
+cpio_dir=$outdir/${tarfile%/*}/.tmp_cpio_dir
 
 dir_list="
 include/