Here is the command to dump linux kernel, suppose "ddd" is the image name, "2097152" is the digit number of 0x200000 in hex, may not be exactly the same of the actual length, but it works.
# nanddump -b -o -f ddd -l 2097152 /dev/mtd3
# nandwrite /dev/mtd3 ddd 0 (I have not test it yet)
