It seems those u-boot files may not be needed after all.
I tried booting the latest current arm64.img without burning any
u-boot to the sd card but this time with a serial cable attached
and I realised that it does start to boot, it just doesn't bring up
HDMI so I wasn't seeing the boot messages.
I don't get as far as the login prompt tho. I get a kernel panic when
its initiating the PCI it seems:
https://controlc.com/35294c1c
I've sent this log to Jared McNeil but I'm not sure he even has a Nano
so its looks like we could be out of luck. Despite the existence of
nano u-boot files (which we don't need) Jared has only tested NetBSD
on the Jetson TK1.