gitignore: add kwb binary images to gitignore file
This adds barebox.kwb and barebox.kwbuart to the list of files to
be ignored by git.
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>