Hi, I just googled uboot marking bad blocks and stumble upon this :
http://wiki.openmoko.org/wiki/NAND_b...cks#Bad_Blocks
Go to the bottom off the page, there's a recipe - Is it what you were looking for ?
Warning : I am not knowledgeable of uboot, I just found this via google - it says it will erase the whole flash - Not sure it is safe!)
nand bad
nand scrub
nand createbbt
dynpart
dynenv set u-boot_env
saveenv