Take look on the PBP wiki page. There’s a paragraph concerning using the NVME as your boot drive. I would suggest using pcm720s uboot, there is an image for SPI.
To answer your question about the 3 uboot images, those would be written to specific areas of the emmc. Those lines from the update script show where they need to go on the emmc. from what i'm understanding of what you're trying to to do pcm720s SPI uboot img will suit you better for booting from NVMe.
To answer your question about the 3 uboot images, those would be written to specific areas of the emmc. Those lines from the update script show where they need to go on the emmc. from what i'm understanding of what you're trying to to do pcm720s SPI uboot img will suit you better for booting from NVMe.