gsm0480: add gsm0480_create_ussd_notify() and *_release_complete()

Add two functions to create USSD messages. Moves and generalizes code from
openbsc. Pending: use the new functions in openbsc.

It looks like _release_complete() should also set trans_id and direction flag;
but since this is moving code from openbsc that is apparently working, just
place a fixme comment and don't change the functionality.

Change-Id: Ia80e32c7105359915bfad3cc5621a1c09caf20f0
3 files changed