openair2: fix abstraction mode for oaisim with MME

When using abstraction mode, layer2_init_UE() and openair_rrc_ue_init()
are called according to first_sync state and only without MME support.

We added the global variable abstraction_flag to allow abstraction mode
when MME is enabled.
Openair2 is also used in lte-softmodem, so it must have a global variable
abstraction_flag. So we moved it out from the main() scope.