Then you can basically upload the next stage (in our case the U-boot SPL) via USB directly to the memory and boot it, using rockusb tools like the opensource rockusbrs. Once the board is ready, one can use a tool like dfu-util and just upload the U-boot proper file from the host computer to the board, and SPL will boot it. Once U-boot proper boots, one can use any network protocol like TFTP to get the kernel, and boot with a file system from e.g. NFS.