Implement a 'v2-reconfigure' command

[23Skidoo]

For old-build we have a useful reconfigure command (originally implemented by @ttuegel), which is like configure, but instead of fully overwriting the old settings file, it mappends the new flags on top of the old ones and records that. I think it'd be useful to have this for v2-build as well.

/cc @bugthunk

[emilypi]

I would like to take this on if possible. Mind if i assign to myself?