I'll answer my own question: I found U-Boot
README.mx28_common which says to create a partition 1, starting at sector 2048, of size 1 MB, with type 0x53. E.g.:
sudo fdisk /dev/sdc
o # to erase partition table
n # new partition
p # primary
1 # partition 1
2048 # start sector 2048
+1M # 1 MB size
t 1 # change type of partition 1
53 # type 0x53
w # write the partition table to the SD card
Then dd the file onto that partition.
sudo dd if=u-boot-imx28evk.mxsboot-sdcard of=/dev/sdc1