Depois de instalar as ferramentas adequadas do uboot na máquina host, consigo obter a imagem initramfs do AARCH64 com êxito.
Cmd:
sudo apt-get install u-boot-tools
root@OptiPlex-790:~$ mkimage -A arm64 -O linux -T ramdisk -C gzip -d init.cpio.gz initramfs.uImage
Image Name:
Created: Tue Sep 20 18:14:33 2016
Image Type: AArch64 Linux RAMDisk Image (gzip compressed)
Data Size: 10240 Bytes = 10.00 kB = 0.01 MB
Load Address: 00000000
Entry Point: 00000000