[oe] Policy on tar.gz -> tar.bz2 -> tar.xz

[Marko Lindqvist]

If one changes compressions of the package to be fetched, what about
old versions of the package:

- Never change compression of something people may have already
downloaded with different compression, even if that means bigger
download for new downloads
- Do not change compression of old versions if they live in version
specific file, but if SRC_URI is in .inc, keep it so even with new
compression, affecting also old version recipes
- Always make future downloads with best compression even if that
means redownload for those who already have downloaded with old
compression.


 - ML