bsc: Enable force-combined-si on nanoBTS by default

Some nanoBTS firmwares (if not all) are known to not work properly with
SI2ter. If BSC enables SI2ter through RSL, SI3 bit announcing SI2ter
available will be forwarded by nanoBTS to MS, but will still only send
SI2 message instead of expected SI2ter during TC=5 (see GSM 05.02 sec 6.3.4 "Mapping
of BCCH data"). As a result, some MS won't allow registering to the
network.

To avoid this kind of scenario, enable force-combined-si by default on
nanoBTS while still allowing to overwrite the feature through VTY. Other
BTS models are kept with force-combined-si disabled by default as
usually, since they seems to be working fine when SI2ter is enabled.

Related: OS#3063
Change-Id: Ide6e8967de0eedc9e2bcaf4414aaa537b009d72d
7 files changed