If I understand this process right:
going through linked OpenWrt guide you will have correct u-boot in emmc.
files that guide shows after decompressing .img to USB flashdrive looks like recovery image + audio drivers + kernel uImage + initial root partition image.
init=/etc/init root=/dev/mmcblk0p1 rootfstype=squashfs rootwait"
That looks like instructing board to look for rootfs on emmc partition 1 (boot: normally uImage + bluecore.audio + .dtb only in there)
I assume it will boot having rescue.root.emmc.cpio.gz_pad.img as rootfs. Then it will do its thing preparing emmc partitioning and contents for actual, proper boot.
- Waiting for OpenWRT burning，after finished，W2 will reboot and go into OpenWRT system.
next reboot - you have everything set right and you do not need USB drive to help you booting.
from now on updates look similar to how you flash android ROMs on your smartphone:
boot to recovery and select file to write to partitions device is using during boot process.
“tab” or “esc” (depending on guide) while turning w2 on looks like the way to initialise recovery instead of normal system boot.
it appears recovery is hardcoded to look for install.img on root of USB drive (no need to select update file manually - just stick it in)
let it process what is needed and possibly reboot back to new system