configkey: boot

{
    "boot": {
        "img": "img/boot.img"
    }
}

Usually a "boot.img" property that has a value set as filepath, is found in base images. When you bootstrap a variant out of this base, and boot.make = true in the config tree, the img will then be unpacked and placed inside the variants dir. The variant's config will also have boot.img expanded and looks something like this:

{
    "boot": {
        "img": {
            "ramdisk": "img/boot/ramdisk", 
            "kernel": "img/boot/boot.img-zImage", 
            "second_size": 0, 
            "dt_size": 534528, 
            "pagesize": 2048, 
            "tags_offset": "0x00000100", 
            "second_offset": "0x00f00000", 
            "cmdline": null, 
            "base": "0x10000000", 
            "dt": "img/boot0/boot.img-dt", 
            "ramdisk_offset": "0x01000000"
        }
    }
}

This allows you to customize the kernel and ramdisk directly inside the variant's file system. And then when you compile your variant (using make), it repacks the img again, including all your changes.