Editing boot.txt to boot from usb drive? (raspberry pi 3B+)

[Toric]

How would I go about editing boot.text on an SD card that does not boot on its own? I would like to put the bare minimum on the SD card, certanly not enough of an OS to boot, install uboot-tools, and edit the file. Can this be done from a host system instead? How would I go about doing this? I am using archlinux on the hostsystem as well, if that matters.

I already tried no SD card and only putting bootcode.bin on the SD card, to no avail.

[schard]

AFAIK, you only need the /boot directory to be available on the SD card.
You can outsource any other directory, including the root directory to another device.
I don't know about any "boot.txt" file, but there is a "cmdline.txt" file on my Raspi2 which I use to customize my system. There you can change the "root=" kernel parameter to whatever applies to your system.