This adds code to handle CBSP (Cell Broadcast Service Protocol)
from the CBC (Cell Broadcast Centre), as well as BSC-internal data
structures for scheduling the various SMSCB on the CBCH of each BTS.
There are currently one known shortcoming in the code: We don't yet
verify if keepalives are received within repetition period.
Change-Id: Ia0a0de862a104d0f447a5d6e56c7c83981b825c7