Thank you hexdump for the suggestion.
I don't have another keyboard, but it worked by keeping a key pressed on the keyboard while powering on the box
And when I do this and the keyboard works, the output to the hdmi is much slower than when the keyboard is unrecognised.
Also FYI, I noticed that the wired ethernet is not working with this u-boot. I tried to use dhcp, but it gives errors about not being able to negociate network speed even if there is link on the cable.