This is because the bl checks if the new fw hwid from tftp is matching your hwid of current fw on flash and you flash is corrupted... So this would also not match if you have a bl that searches over tftp for v5 fw....
Why not write the fw as you wrote the bl to the flash?
You will see if you have the v5 fw in flash you could also flash a new v5 fw with tftp.
Here is a thread that describes the same problem...