Can I suggest an experiment?
You have a usb->emmc adapter so it should be 'easy'
On emmc, save mbr, save 1st 16M, write /dev/zero to 1st 16M, restore mbr
(using dd,, bs-1M count=16 OR dd,,, bs=512 count=1 OR 34 if gpt)
This forces use of uboot on SD, maybe will get past flashing lights
Since uboot priority is SD, rename boot.scr to boot.scr.bak OR extlinux.conf to extlinux.conf.bak (on SD)
If it boots, check with lsblk to see what is /, emmc or SD
But it probably won't,,, this implies that distro is not good and uboot not at fault
You have a usb->emmc adapter so it should be 'easy'
On emmc, save mbr, save 1st 16M, write /dev/zero to 1st 16M, restore mbr
(using dd,, bs-1M count=16 OR dd,,, bs=512 count=1 OR 34 if gpt)
This forces use of uboot on SD, maybe will get past flashing lights
Since uboot priority is SD, rename boot.scr to boot.scr.bak OR extlinux.conf to extlinux.conf.bak (on SD)
If it boots, check with lsblk to see what is /, emmc or SD
But it probably won't,,, this implies that distro is not good and uboot not at fault