bsc: implement ttcn API and cfg for A5/4

Implement tools for OsmoBSC a5/4 support testing:

- in f_cipher_mode() and f_check_chan_act(), expect Kc128 key as
  appropriate, using recently added g_pars.encr.enc_kc128
- osmo-bsc.cfg: allow a5/4

Related: SYS#5324
Change-Id: Ifa48a8498dde7d04fb29f497013bdb5a1e5f3597
3 files changed