If we don't do this, the deploy sstate object contains an every
increasing number of modules tarballs and kernel images, one per
execution of "-c deploy -f".
Cleaning the directory before we start makes things much tidier.
(From OE-Core rev: 3ac195bbd4ced24e6a132cfbe5e175b95fc4664b)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>