You mean with a freshly flashed SD card, and USB device and eMMC removed, it does not boot from there? Similarly bad. Of course, erasing petitboot is otherwise the mandatory step, but I was actually hoping that our changes allow boot via petitboot, regardless whether from USB or SD card or eMMC. Before erasing petitboot, test first holding the MASKROM button. Probably something else went wrong.
To flash mainline U-Boot to SPI, which should enable reliable USB boot:
apt install linux-u-boot-current-odroidhc4
source /usr/lib/u-boot/platform_install.sh
write_uboot_platform_mtd "$DIR"
Interesting would be however whether a fresh image with above change in boot scripts only boots from USB as well with petitboot.