do we really need root "overwrite" option?

[jrollins]

I've been wondering if we really need the root "overwrite" option in debirf make. This option does a debootstrap over the existing root if it already exists, without first clearing it out. I've never actually used it, I'm not sure I ever would, since it would leave the root in a potentially ambiguous state, and I think it might be a little confusing. I propose we remove the option altogether.