Great!
uboot options like fdt_high tend to be quite poorly documented, so it was good that you found the problem. Have to admit I only usually find options like that by browsing the uboot source code - which I wouldn't suggest anyone else does!
Usually there are updated uboot available for these boards, had a quick browse last night - and doesn't seem one done directly for the odroid-xu, there are for most other odroid boards, and its possible one of the others also works on the XU (probably needs same cpu and basic layout of the board).
Booting with the flattened device tree is the new way of describing hardware, its actually far better than the old way, far more flexable. So the change to that is good. Most uboot scripts can take both initramfs and dtb options, and both are needed at times (e.g. if you root filesystem is not known to the kernel). Anyway doing this with the old uboot enviroment variables was quite a hassle, its easier with more modern uboot when you can script.