mme: Add code to derive NAS token from NAS ul_count

NAS Token is derived from kasme and NAS ul_count as specified in 3GPP TS
33.401 A.9, and its LSB 16 bits passed to the network when mapping the GUTI to
RAI+PTMSI+PTIMSI_SIG.

Take the chance to move guti2rai_ptmsi() up to before the place it is
used.
Change-Id: I5e6003a2fe3e74cc93cfe4a288e6c114aa288d0b
6 files changed