[Buildroot] USB on rock64

[no-one-you-know]

> Have you tried with other devices? USB3 flash drives don't always work well in
> USB2 ports (yes, they should, but they don't...). A mouse is often a much better
> device for testing.

Hello - thanks for responding.

Yes, I tried with a USB2 flash drive and a USB keyboard.  Nothing detected.
I'm reasonably confident it's the kernel rather than a missing package because I used the same uboot/kernel with a different rootfs and have the same behaviour.