Just some notes you might find useful, not an answer…
When you flash, even if you are booting external media, then in some way eMMC still has to point to the SD card. In some cases the pointer involves the “
/boot” content of the eMMC even if rootfs is on SD card. In other cases you might end up using the “
/boot” content of the SD card. It is likely that in both cases there would be a minor difference in
extlinux.conf, and that if you edit the wrong one, then your edits will be ignored.
If you are using U-Boot (and TX2s do use U-Boot), then you can interrupt boot at the kernel selection stage via serial console. You’ll note that each kernel entry has a “
MENU LABEL”. You can alter that menu label without changing what the entry actually does, and you’ll see that label when offered a chance to select which kernel to boot. If you change the label on eMMC to “
primary kernel eMMC”, and on the SD card change the menu label to “
primary kernel SD”, then you’ll be able to watch and know which extlinux.conf is being used…SD card version, or eMMC version.
There is a chain of boot content, and part of that chain will always be on eMMC. You cannot put absolutely everything on SD.