One interesting thing I discovered today is that you can actually boot from a flash drive, if you don't have a SD Card! In my case, I would have to buy a SD Card and had a flash drive laying around so I thought I might just give it a try, worked on an Arm Cortex-A53 CPU. I used u-boot-s905x-912 for .ext and meson-gxl-s905x-p212.dtb.