Allow the user to use wrap as part of the bootstrap process

[MaxHearnden]

This adds support for wrap in three situations:

1. Using rootfs.py -w
2. In a prepared environment containing distfiles, seed and steps (as sub directories)
3. In a clone of the whole git repository with distfiles (Although it does overwrite tracked files within the git repository)

For 2 and 3, wrap can be used by copying seed/after-wrap.kaem to seed/stage0-posix/after.kaem (symlinking works as well) and running bootstrap-seeds/POSIX/${ARCH}/kaem-optional-seed from within the seed/stage0-posix directory

This replaces https://github.com/fosslinux/live-bootstrap/pull/339