The @Dlonk method detailed in Reflash using SD card - #4 by Dlonk with the primary difference being the use of an internal SD card and the possibly desired simpliciity of the actual install being a dd of the disk image onto the L5 EMMC. The card must have enough capacity to hold both a bootable pureos and the image to be flasfhed to the L5.
@elkingrey please note that the L5 can boot from the SD card. I don’t know why it’s not possible to boot from the USB.