What platform are you talking about? Please clarify in subject. Assuming this is for cc6ul:
your u-boot needs to be in internal FLASH. You can't boot entirely of SD card. The rootfs and kernel could be on SD card.
something along those lines should work:
> => setenv mmcroot /dev/mmcblk1p2
> => setenv mmcdev 1
> => dboot linux mmc