You can always grab the firmware yourself from the Raspberry pi next repo and update it in your system partition, then you will be able to usb boot.
What is the best option to do this? I am still reading and learning, but as far as I know, Libreelec contains the master branch of the firmware and not the "next" branch. My pi is ready to boot from USB. I also tried to just put an update bootcode.bin on to the flashed USB, but this only gave me the rainbow start screen. So my thought was that the cmdline.txt had to be updated as well. But I don't know to what I should update this.
As far as I know, at initial startup, the partitions will be created. So what is the best way to handle this?
This is my cmdline which works with my Rasbian image. Not for LibreElec
dwc_otg.lpm_enable=0 console=serial0,115200 console=tty1 root=/dev/sda2 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait plymouth.enable=0