unmkinitramfs: Make it compatible with Busybox cpio

The miniramfs project creates smaller first-stage initramfs images that chain into the initramfs images created with initramfs-tools. It uses Busybox cpio to unpack the second initramfs, which stopped working entirely since we split the contents across an uncompressed and compressed cpio archive.

miniramfs would benefit from being able to run unmkinitramfs, but currently that doesn't work due to the use of GNU cpio options that Busybox cpio doesn't implement. Fix that.

Extend the unmkinitramfs test to cover all 3 implementations (3cpio, the default cpio, and Busybox).