1: cannce bootz ( let uboot recognize Image as uImage)
mkimage -A arm -O linux -T kernel -C none -a 0x10008000 -e 0x10008000 -n 'TestImage' -d arch/arm/boot/Image arch/arm/boot/uImage
2: bootm 12000000 - 12c00000
It seems there no obviously boot time reducing between using zImage & Image