Custom rootfs support

[colemancda]

Hi, I am building my own distro rootfs with Buildroot and wanted to use Cadmium to build image with my custom rootfs. I think Arch was using a similar method, but I want to use supply a rootfs.tar and let Cadmium do the rest like it currently does for Arch and Debian.

[Maccraft123]

I've thought about this but it would entirely break all rootfs installation except extracting the image,
feel free to propose a nice looking solution