uboot-imx

cmd: fit_image: Add default property for FIT format scripts

When sourcing a FIT format script, if we've not been told the unit name
to use, look for a default property at the root of /images to work out
which unit we should use.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>