FROMGIT: kheaders: rename the 'cpio_dir' variable to 'tmpdir'

The next commit will get rid of the use of 'cpio' command, as there is
no strong reason to use it just for copying files.

Before that, this commit renames the 'cpio_dir' variable to 'tmpdir'.

No functional changes are intended.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Bug: 354775288
Bug: 368667510
Bug: 291611617
Change-Id: Ifccf20c43cefbed6eeecfec4eb12ca77ed2942bf
(cherry picked from commit 28186b227b316f5be2a9d7343f32e0457ffdd953
 https://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git

 for-next)
[elsk: Resolve minor merge conflict due to
 28fa7afc "FROMLIST: kheaders: dereferences the source tree".]
Signed-off-by: HONG Yifan <elsk@google.com>