wic: Ensure internal workdir is not reused

If a path is specified for the internal wic working directory using
the -w/--workdir argument then it must not already exist. Re-using a
previous workdir could easily result in rootfs and intermediate files
from a previous build being added to the current image.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>