In manual image creation, if you need 128KB of padding between the bootloader and the kernel, you often have to dd zeros into a file, concatenate files, and calculate offsets manually. It is error-prone and fragile.
The output sdcard.img can be written directly to an SD card with dd . genimage
GenImage is a command-line utility that generates filesystem images from a given directory tree. Unlike simple archivers, it creates partition-ready image files (e.g., rootfs.ext4 ) that can be directly flashed to an SD card, eMMC, or NAND flash. In manual image creation, if you need 128KB