Instead of continuing to add more and more functionality to the
bsc_hack binary, we should have the new SGSN code run as a separate
executable.
After this commit we now build a 'osmo_sgsn' executable, using its
own osmo_sgsn.cfg config file.
However, the SGSN is not yet functional, mainly due to the fact that
the BSSGP and GMM code are written with the assumption that there
is a msgb->trx->bts and the according 'sturct gsm_bts' data model
around - which clearly is no longer the case outside of bsc_hack.
