combo-layer: dont use bb.utils.rename

Bitbake may not be configured, and bb isn't imported anyway.

Instead just use os.rename(), and take the filename from the file object
instead of duplicating logic.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 528f4fb3)
Signed-off-by: Steve Sakoman <steve@sakoman.com>