To accomodate the ip.access nano3G without having to recompile, make the
RAB Assignment's Transport Layer Address IE's format configurable, in both
osmo-msc and osmo-sgsn.
The long term perspective is to somehow detect which address encoding a given
3G cell prefers, but for the time being just configure it globally and set each
ue_conn_ctx to the global setting upon creation.
Add VTY command to iu_vty.c, with args passed to iu_vty_init() to insert
at an arbitrary VTY node, so that it can be used both for osmo-msc and
osmo-sgsn. Add generic iu_vty_config_write() to write out the config.
Call iu_vty_init() from msc_vty_init(); msc_vty_init() is in libmsc,
hence linking of osmo-msc now needs libiu to come after libmsc, so move
that further down.