hnbgw: UE context: add handling by tmsi identification

To prepare for an upcoming commit that accepts TMSI identification upon UE
Register Requests:

Add tmsi arg to ue_context_alloc().
Add ue_context_by_tmsi().

This is aimed at the ip.access nano3G femto cell, as it apparently feeds
whichever identification the UE sends through to HNBAP (TMSI+LAI, pTMSI+RAI),
instead of an IMSI as expected.

See the upcoming commit that enables accepting TMSI identities for further
detail.

Change-Id: I138458443319cc4cbea5ee7906cf5dd72d582130
3 files changed