If you want to flash uboot I suggest you read carefully the help page provided in the help drop down menu of the fvdw-sl console
Attached a zip archive with the file you will need (u-boot-5big2-spi-3416-1517.bin). It is a copy taken from a 5big2 with spi flash. The .md5 file is a file you could use to verify the md5sum of the .bin file to be sure it did go corrupt in the transfer process from this forum to your pc.
You have to put the u-boot-5big2-spi-3416-1517.bin and md5 file in the tftp folder of the fvdw-sl-console to be able to select it.
Be sure to select the u-boot-5big2-spi-3416-1517.bin file when you perform the flash