Ah, now I answered here: Odroid HC4 booting from SSD - #39 by MichaIng
Not that enabling USB support via petitboot is not preventing is from going anywhere. It is just some convenience feature so that users do not need to erase the SPI, respectively flash mainline U-Boot to it first, e.g. by booting via SD card first or one of Hardkernel’s images. With mainline U-Boot it should all work well anyway.
With additional steps, all is possible, this is just to attempt to avoid the need to those additional steps and enable USB boot just OOTB, as petitboot is what Hardkernel’s SBCs with SPI are delivered with.