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

This reverts commit 774d4de3.

This is necessary to also revert commit 28fa7afc ("FROMLIST:
kheaders: dereferences the source tree"), which is no longer applicable
and it causes conflicts with upstream merge. Note the upstream version
of this FROMGIT will land via merge instead, so no worries.

Cc: Yifan Hong <elsk@google.com>
Change-Id: I38a5e9c62177a4a707506aab284901f02e39cbf6
Signed-off-by: Carlos Llamas <cmllamas@google.com>