Since right now I don’t care about saving data, I used Libre Computer AML-S905X-CC eMMC Flashing Steps from Linux with pyamlboot - #38 by Peter_Meigs advice to erase the eMMC boot block to force it to boot from uSD, like:
sudo dd if=/dev/zero of=/dev/mmcblk0 bs=1M count=1
But how do I boot from uSD if eMMC is also bootable? Can I force the potato to try uSD boot before eMMC boot? Or is there a boot selector that can be included by default in the eMMC image?