MA-8202 [Android_MX6DL_ARD] Fastboot: Failed to boot system if update android_root.img based on NAND boot. 100%

For sparse system.img, USB can trasfer system.img through spliting image into two sparse images.
For android_root.img, the second part will overwrite the first one.
It is a workaround to enlarge CONFIG_USB_FASTBOOT_BUF_SIZE to hold system image whose size is more than 400M.

Signed-off-by: zhang sanshan <b51434@freescale.com>