Investigate extracting the rootfs with symlinks preserved

[fe-wickham]

I misspoke earlier. -p is for preserve permissions, which is default when run as superuser (we do want permissions preserved).

I think it is preferable to keep the symlinks rather than dereferencing them, but I don't know if dereferencing causes any practical problems. Judging by reports so far, it doesn't appear to be an issue. My thought is that if symlinks are there in the original Ubuntu base archive, there is probably a reason, so we should keep the symlinks.

I would think using the same tar arguments that we use in build_rootfs.sh would work correctly, though, as I don't recall having any issues with that before.

We can verify if we are removing symlinks by running find with -type l on the original Ubuntu base directory and our modified rootfs.

Originally posted by @tvannoy in #24 (comment)