Discuss xz support for platforms

[mobileoverlord]

This is required especially for windows support.

[mobileoverlord]

After a long trial and error period with @fhunleth we decided we needed to keep toolchains in xz compression which is not supported by erl_tar. This requires us to continue to shell out to tar. I am updating the description of this task to redirect it towards a conversation about xz and windows

[fhunleth]

The reason for not using erl_tar was that it failed to extract the toolchain tarballs. The initial problem was that we are using GNU Tar to create the tarballs. Upon review of the erl_tar docs, we started specifying --format=ustar when calling GNU Tar. This created the additional problem of restricting path lengths too much to include a couple files for the crosscompiler. An attempt was made to make a root directory-less tarball so that the path length constraint could be met, but this prevented Buildroot from using the toolchain.