install to-existing-root: don't blow away /boot until later

[cgwalters]

In install to-existing-root, we can delay blowing away /boot until very late in the process - specifically until we've pulled the container image for example, which may hit problems.

It'd make us overall more robust to being ctrl-c'd in general and we could even have a last minute extra warning + delay there as it's really that process that's the destructive thing.