The boot loader is placed on the Boot Partition(Hidden Partition) in the eMMC memory. (C1 model doesn't use the Hidden partition).
When it is corrupted or you want to use the eMMC to other different board, you must install the proper boot loader in the eMMC.
Note that you must have a micro-SD card to do the recovery process.

ODROID-XU3/XU4 eMMC Recovery