libupnp: add package libupnp

This patch adds libupnp package to Buildroot. libupnp provides developers
with an API that are compliant with version 1.0 of the Universal Plug and
Play Device Architecture Specification.

http://pupnp.sourceforge.net/



Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>