Add basic support for CBCH / SMS-CB (Cell Brroadcast)

We can now configure the pyisical channel types for CBCH either in the
CCCH+SDCCH4 or in the SDCCH8 chanel combination.

Depending on whether a CBCH exists on the BTS, we also generate the SI4
with matching CBCH channel description to notify the phones of the
existance of the CBCH.

There is now a VTY command how a SMS-CB message can be sent to a given
BTS.

We do not yet have any logic at all for actual scheduling of multiple
CBCH RSL messages towards one or multiple BTSs yet, though.
move gsm_bts_get_cbch() to gsm_data_shared() as its needed in osmo-bts