iwinfo: build with nl80211 backend only and make shared

iwinfo: build with nl80211 backend only and make shared

Drop support for building the obsolete broadcom-wl backend and always
forcibly enable the nl82011 support. This allows us to make the package
shared again since no target specific compilation is happening anymore.

This will solve various repository coherency issues related to unavailable
libiwinfo versions in the long run.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: maurerr <mariusd84@gmail.com>