linux-libc-headers: make compression format configurable
As of the 3.13 kernel bz2 compressed tarballs are not available. To support
older header tarballs, and newer ones that require the 'xz' compressed
bundles, we can break out a variable that allows versioned libc headers to
select the archive format that works.
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
