Branch 1.8.x of libupnp is not compatible with branch 1.6.x so add a
dedicated package and make it depends on !BR2_PACKAGE_LIBUPNP as
suggested by Thomas Petazzoni and Arnout Vandecappelle during review
of "libupnp: add 1.8.3 version" patch.
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Thomas: fix the dependencies of the Config.in comment.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>