mini2440: changed env size to 128k

mini2440 has SAMSUNG K9F2G08U0C on board, and it's block size is 128k

SMDK2440 # sa
Saving Environment to NAND...
Erasing Nand...
Warning: Erase size 0x00010000 smaller than one erase block 0x00020000
         Erasing 0x00020000 instead

nand0: MTD Erase failure: -22

Writing to Nand... FAILED!

Signed-off-by: Fudong Bai <fudongbai@gmail.com>