[gentoo-alt] Allow non-interactive bootstrap to be stopped at arbitrary stages

[Benda Xu]

Hi Sam,

Sam Pfeiffer <sammypfeiffer@×××××.com> writes:

> Benda, I'm sorry for the noise I'm causing. I just want to improve the
> experience for the next person that tries to use this amazing project
> by taking advantage of already implemented stuff. And also just having
> this discussion is good to have more information archived to google
> about Gentoo Prefix and its bootstrap process. I think.

No problem at all for me.  Keep up with good CI work.

Benda