If no cache file name is given a default from class variable is used,
like before.
(Bitbake rev: 2602a312818f564961de7dfa63c429d45ff9e5ac)
Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>