OsmoBTS doesn't support SMSCB "Default" Message
There's a special SMSCB message type which is sent by default when nothing else is to be transmitted on the CBCH. OsmoBTS doesn't implement this so far.
cbch: Refactor get_smscb_block() / remove smscb_msg.next_seg
There's no need to keep around a pointer to the next segment
in a SMSCB message. The way how the multiframe structure is
laid out (and how the tb number works), we can use the result
of a modulo-division on the frame number to determine which
of the segments/blocks inside a SMSCB message (page) we have
This also acts as a simplification in preparation of support
for the SMSCB DEFAULT type.
cbch: Implement support for DEFAULT message
The BSC can not only send us each to-be-sent message separately, but
it can also configure a DEFAULT message, which is then to be sent
instead of the NULL message. Let's add support for this