We actually do use Grub in the Fedora images, and can as such boot anything.
Only current problem is, that we do not yet have the screen working at this early phase,
so you have to use a debug cable to do the selection in Grub.
As a more quick and dirty solution, you could simply overwrite uboot on the sdd card with
zeros with dd, and reboot, then it boots from emmc. When you want to boot from SDD card,
do wite the uboot.bin back to the SDD card and reboot.
Only current problem is, that we do not yet have the screen working at this early phase,
so you have to use a debug cable to do the selection in Grub.
As a more quick and dirty solution, you could simply overwrite uboot on the sdd card with
zeros with dd, and reboot, then it boots from emmc. When you want to boot from SDD card,
do wite the uboot.bin back to the SDD card and reboot.