buildroot

janus-gateway: make sofia-sip a optional dependency

Currently janus-gateway requires sofia-sip in the build system,
however this is unessasary. This patch makes the sip gateway plugin a
option. It also creates a menuconfig for janus gateway for the
following patches that enable selecting more optional plugins.

Signed-off-by: Adam Duskett <aduskett@codeblue.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>