feat(cmd): Add command to unpack rootfses

[craciunoiuc]

Currently blocked on these, so in draft:
unikraft/go-archivefs#10
unikraft/go-cpio#2

erofs one worth discussing if it can be done cleaner

Closes: TOOL-1029

[jedevc]

Copying from https://linear.app/unikraft/issue/TOOL-1029/add-unikraft-image-unpack#comment-edc8098e

Hm. So I see unikraft/go-archivefs#10 but why do the unpack instead of just providing an implementation of fs.FS? Then we have a generic unpack function which just takes an fs.FS in one place.

If everything is using an fs.FS, then for example, it makes it super easy to translate a cpio to an erofs file, without needing to unpack to disk. Essentially like the tarfs already does.

Remember there's an extension of this as TOOL-1030, which would mount the fs.FS as a FUSE filesystem.

Wait. Doesn't it already do this? https://github.com/unikraft/go-archivefs/blob/a06ac4112571c729baf5bcafdcf1f255b82796a7/erofs/erofs.go#L41-L53

This is the erofs file → fs.FS reader?

[craciunoiuc]

Hmm, right, I think I forgot about that in a way, I'll have to revisit