update u-boot for a20 to speed up the booting process
delete u-boot-nand.bin, we doesn't need it
update 3.4.61+ modules
Revert "update u-boot for a20 to speed up the booting process"

This reverts commit cc73209b5147a6cacdf5cdf3b725fadb18eeb457.
use new uboot to speed up the boot process