When constructing the path to the boot image, don't assume the NBI's are in a root level directory

[vmiller]

This patch constructs the relative path to the boot image by subtracting the tftprootpath from the full path. Since tftprootpath is passed into bsdpy at runtime, it makes sense to use this instead of making any assumptions about directory depth.

[standardtoaster]

I'm checking out this project, and came across your pull request. In trying to review this request, I noticed that your commit messages lack context, especially outside of this pull request. Somebody (like me) reading the commit logs later would have a bunch of trouble figuring out why you're doing something, the what being reasonably obvious from the code.

There's some good tips about writing commit messages at https://robots.thoughtbot.com/5-useful-tips-for-a-better-commit-message