FYI,

You can reset the u-boot bootdelay from 0 if you can get to a linux prompt.

dd if=/dev/mtd2 of=env.blob
vi env.blob 
Find the string bootdelay=0 and replace the 0 with a 5
dd if=env.blob of=/dev/mtd2

The other way to do it is to create a boot.scr, add it to your mmc boot partition, and add a
setenv bootdelay 5
saveenv

To the top of that script.   For more details see http://wiki.gumstix.org/index.php?title=U-Boot

--David.