The Texas Instruments AM335x SoCs have a two-stage boot process. The first stage loader, also known as MLO is loaded by the ROM code. The MLO loads the second stage loader from the same medium.
The TI AM335x boards in barebox are covered by the am335x_mlo_defconfig for the MLO and am335x_defconfig for the regular barebox image. The resulting images will be placed under images/:
Some boards come in different variants, make sure to pick the correct one.
The regular board images can be started from another bootloader, see Starting barebox. The board should provide update handlers to update (or initially install) barebox on SPI NOR or NAND, see Updating barebox.
To start barebox from SD/MMC prepare a card with a FAT filesystem. Copy the MLO file for your board to the card and name it MLO. Copy the regular image for your board to the card and name it barebox.bin.