SD readers aren’t equal. Our one is integrated into a USB hub and uses USB interface. u-boot would be already able to boot straight from SD card on L5 if the reader used the MMC interface of the SoC, but we’ve got only two of those and they’re already taken by eMMC and WiFi. So, u-boot needs to learn how to be a USB host on L5 and then access the mass storage drive in order to boot from it.