> This currently supports installing either Gentoo or Funtoo only from stage 3 tarballs. It does not automatically grab a bootstrap tarball. This is at least in part because I don't have ARM hardware to test on and it is unclear to me what the appropriate installation sources would be for that architecture (putting aside the question of whether anyone would actually install this on ARM at all). Furthermore, an ordinary Gentoo installation necessitates that users explicitly select an installation source so it seemed reasonable to replicate that in the crouton context.

That seems to map well to the `-m MIRROR` parameter, which could specify the location of the stage 3 tarball to be downloaded.  I'm sure you could find some reasonable defaults if MIRROR isn't specified.