Unpacking:
Create a folder for example "initramfs", copy the file initramfs.cpio.gz in it. Unzip it:
$ gunzip initramfs.cpio.gz
Extract the cpio file and remove it after you're done:
$ sudo cpio -i -d -H newc --no-absolute-filenames < initramfs.cpio $ rm initramfs.cpio
Packing:
$ sudo find . | cpio -H newc -o > ../initramfs.cpio $ cd .. $ gzip -9 initramfs.cpio