patch: reorder BOOT_TARGETS to sd, nvme, emmc, usb

With the current u-boot on rk3588, it will only try loding a bootflow from the sd card and then fall back to emmc. Only if both devices lack bootflows (i.e. no boot.scr, no extlinux.conf) will other devices be tried.

This patch reorders this search, so that it looks for bootflows in the sd card, then nvme, then emmc. This way, a person can provision their emmc with reform-flash-rescue, and that can boot from SD or NVMe while having dedicated /boot directories on those devices. It's nice to have one predictable place to load u-boot, and just load bootflows from the target devices. Trying to share emmc /boot among different OSes can lead to conflicts (as different update scripts attempt to manage vmlinuz-* files and so forth).

With this u-boot flashed to my emmc, I was able to do a standard Alpine installation with 'setup-alpine', add an extlinux.conf, and get a working system without having to share any filesystems between distros.

Scanning the USB tree adds a couple extra seconds of boot time, so we leave it for after emmc to avoid the probing delay.