[U-Boot] Default script entry using FIT format scripts

[Alex Kiernan]

I'm trying to get a build going without the legacy image format, but I
need boot scripts (and I'm using distro boot). I'm coming unstuck when
trying to boot one as there's no way to pass in the FIT unit name to
source with distro boot.

Would adding a default property inside /images (like we have inside
/configurations for bootable images) be a reasonable approach? Trying
to add the full configuration machinery feels over the top (though I'm
wondering if down the road we want signed scripts if that might
preclude it).

-- 
Alex Kiernan