- Official Post
Not sure what you mean exactly. U-Boot has search procedure for extlinux.conf stored in environment variables. This is the same for all platforms, not only Allwinner. You can change them at runtime with setenv and store them with saveenv.
Still, having system on USB dongle and having U-Boot on SD card is very inconvenient for the end user.
USB startup support is needed primarily for those who use LE and Armbain from eMMC or on SD card and want to help quickly test new versions, but do not have free SD cards. USB flash drives are much easier to use (you do not need a card reader on your PC to write) and significantly cheaper than SD cards. Many have old USB flash drives that they can easily use to run and test any system (no need to change the SD card in the device, plug in the USB flash drive and test).
I think I found where to make patches for u-boot. Now I will test at myself.