You could be getting an illegal instruction error if you overwrote part of uboot with the kernel image ie the partitions won't hold the data. then you are calling for bits that you overwrote.
And the device tree stuff changes things too.
I ran into those issues with the pogoplugs.
I forgot the uboot commands off the top of my head but check the size of your partitions vs the size of what you are loading. Uboot will work even if you overwrote some of the bits if you don't call those sections of the code.
I ended up repartitoning and changing all of the memory addresses. Which is kind of a pain.
I hope that helps a little.