xz: Update LICENSE variable for xz packages

Update LICENSE defined for xz packages to match the license
information provided in the xz COPYING file.

The License information from PACKAGERS file of xz mentions
packages with lzma files are in public domain.They ask to
use GPLv2+, if only it's not possible to mention "PD and GPLv2+".

Include PD license with GPLv2 to packages with lzma content:
xz-dev package contains lzma header
xz-doc package contains lzma man pages
xz packages contains lzma binaries

Links: https://github.com/tukaani-project/xz/blob/v5.4.6/COPYING
       https://github.com/tukaani-project/xz/blob/v5.4.6/PACKAGERS



Signed-off-by: Bhabu Bindu <bindu.bhabu@kpit.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>