在构建FIT镜像时，需要将ramdisk放在initrd子节点中，并在bootargs中指定initrd的位置和大小。

示例代码：

/dts-v1/;

/ {
    description = "FIT image with Linux kernel, DTB and ramdisk";
    #address-cells = <1>;
    #size-cells = <1>;

    images {
        kernel {
            description = "Linux kernel";
            data = /incbin/("zImage");
            type = "kernel";
            arch = "arm";
            os = "linux";
            compression = "gzip";
            load = <0x80008000>;
            entry = <0x80008000>;
            hash {
                algo = "sha1";
            };
        };

        fdt {
            description = "Flattened Device Tree blob";
            data = /incbin/("device-tree.dtb");
            type = "flat_dt";
            arch = "arm";
            compression = "none";
            hash {
                algo = "sha1";
            };
        };

        ramdisk {
            description = "Initial RAMdisk";
            data = /incbin/("ramdisk.cpio.gz");
            type = "ramdisk";
            arch = "arm";
            os = "linux";
            compression = "gzip";
            load = <0x82000000>;
            entry = <0x82000000>;
            hash {
                algo = "sha1";
            };
        };
    };

    configurations {
        default = "conf";

        conf {
            description = "Boot Linux kernel with DTB and ramdisk";
            kernel = "kernel";
            fdt = "fdt";
            ramdisk {
                description = "initramfs";
                image = "ramdisk";
                type = "initrd";
                arch = "arm";
                os = "linux";
                compression = "gzip";
            };
            signature {
                algo = "sha1,rsa2048";
                key-name-hint = "test-key";
                sign-images = "kernel,fdt,ramdisk";
            };
        };
    };
}

在U-Boot中使用命令bootefi启动：

setenv bootargs "console=ttyS0,115200n8 root=/dev/ram0 rw rdinit=/sbin/init"
bootefi 0x82000000:$filesize