gprs_bssgp and gprs_bssgp_util.c also contains code related to send and
receive RIM PDUs via BSSGP and also code to encode and decode RAN
INFORMATION PDUs. Lets move this to gprs_bssgp_rim.c
Change-Id: Icda279452962b06e552cb1361d2a27b7dc8a6b04
Related: SYS#5103
Add bssgp_ns_send callback() to set the transmission path into the
NS library. This allows to use the Gb implementation with
the old NS and the new upcoming NS implementation.
Users of the old NS implementation don't have to set the callback as
the default is the old NS implementation.
Only users of the new NS implementation need to set the callback and
the callback data.
Change-Id: I3a498e6a0d68b87fed80c64199b22395796761b4