You could try this as a starting point:
http://processors.wiki.ti.com/index.php/AM335x_U-Boot_User's_Guide
You may download the TI SDK, build the U-boot for your platform, find the resulting u-boot.img file, and ... copy it to the SD card.
You could try this as a starting point:
http://processors.wiki.ti.com/index.php/AM335x_U-Boot_User's_Guide
You may download the TI SDK, build the U-boot for your platform, find the resulting u-boot.img file, and ... copy it to the SD card.