tools: image-host.c: use random instead of rand

According to the manpage of rand, it is recommended
to use random instead of rand. This commit updates
the function get_random_data to use random.

Reported-by: Coverity (CID: 312953)
Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>