Support make var expansion in pkg_tar

We have a use case where we need to use variables from the toolchain's make vars in order to interpolate paths passed to remap_paths attribute in pkg_tar. Currently this is unsupported and results in the files not being found. Is this something that we'd be open to supporting?

Start by locating the pkg_tar implementation and the remap_paths attribute handling, then trace how the toolchain's make vars are exposed. Done means remap_paths can interpolate those variables and the resulting paths are found.

Written by the indexing model from the issue text.