liblzma is part of xz and we already build it but configure it out. This makes
no sense. Enabling it means we gain multithreaded compression and it speeds
dpkg-deb up massively. It also removes the fork overhead of separate xz processes.
Turning the existing config into a PACKAGECONFIG and turning it on by default
therefore makes best use of what we have available.
The manual RDEPENDS are no longer needed since it uses liblzma which is picked
up by the shlibs code magically.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
