wic: use find_executable in favor of bb.utils.which

wic: use find_executable in favor of bb.utils.which

As bitbake is not ported to Python 3 yet it's better to
avoid using its APIs as much as possible to be able to
test wic under Python 3 at least partially.

Used distutils.spawn.find_executable API in favor of
bb.utils.which to get path of the command to run.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>