Replace offsets to your offsets, found from downstream boot.img.
CONFIG_SYS_MALLOC_LEN - size of early C runtime environment heap.You don't need this, since stock bootloader will load whole u-boot image into RAM. SPL(Secondary Program Loader) splits u-boot in two parts. Put config options not supposed to be changed by user in /include/configs/.*.h files, in *_defconfig otherwise U-boot configuration system comes from linux kernel.
Use guide from Free Electrons pdf or video version Examples