wic: plugins source bootimage/isoimage rename to allow be imported
Python not support importing modules with - so change to _.
(From OE-Core rev: 8ee5b053de4c9222f5a02b1973089e83aa852bca)
Signed-off-by: Anibal Limon <anibal@limonsoftware.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>