I´ve tried to do something similar. I have a nespicase with a pi3 in it. I want to use it sometimes with Recalbox on a SD-Card, but mostly with LibreElec on a USB-Device that is build in the case. I´ve
- activated the USB-mode with Rasbian
- buyed an supported USB-Device (SanDisk Cruzer Fit 16GB)
- formatted it with SD-Formatter
- used the USB-SD-Creator to write the image
- changed the cmdline.txt to "boot=/dev/mmcblk0p1 disk=/dev/sda1 ssh quiet" (I´ve tried it also without the "ssh")
My Pi tries to boot now without SD-Card, but it gives out the error in mount_flash: "could not mount" and "can´t access tty; job control turned off"
Do you have any idea to fix it?