wic: Add workdir argument

This allows the path for the temporary workdir used by wic to be set
when running wic from bitbake or directly from the command line.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2e7314ac)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>