Make sure ubinize-image.sh also works with more simple POSIX Shell and
allow creating complete custom images to be used as ARTIFACT/foo.img
and thereby allow including uImage.FIT, TF-A FIP and what ever else
is required on a specific board.
Fixes: 6c17d71973 ("scripts: ubinize-image.sh: support static volumes, make size optional")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>