Currently unpack just forces sources over the current files. This change
ensures ${S} is cleaned out before sources are unpacked. This resolves
issues seen when upgrading to the new kernel class changes.
Ultimately, this should probably move to base.bbclass but one step at a
time, this solves an immediate problem blocking the other patches from
merging.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
