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: Steve Sakoman <steve@sakoman.com>
(cherry picked from commit a2308800b7eb5215982db7269d74d6292075f19f)
Signed-off-by: Steve Sakoman <steve@sakoman.com>