[rilmodem][WIP] Introducing basic SMS cell broadcast support

Added handling of incoming SMS broadcast messages. Implementation
should enable device to receive emergency cell broadcast messages
but it needs more testing. Currently it has been tested only against
stub.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Merge pull request #27 from jkangas/master

Request for pulling basic SMS cell broadcast implementation to main branch