I use BF548 ezkit and distribution 2011R1-RC1, I can found SD card under /dev after boot and card inserted. Here is the log.
The config/uImage I shared you does not have "MMC block device driver" built into the kernel, it's into module instead. You may build it into kernel or run "modprobe /lib/modules/3.0.8-ADI-2011R1-svn10227/kernel/drivers/mmc/card/mmc_block.ko" after boot up.
Retrieving data ...