Configure complete PDCP in lte-(ue)softmodem

* Instead of partly initializing in read_config_and_init(), everything of
  PDCP is initialized in main() of lte-softmodem
* Omit RC.rrc access in RRU through checking of RC.nb_inst > 0
* Includes FlexRAN, i.e. do not start FlexRAN in RRU
* Function pointers to PDCP functions have to be set explicitly (avoid
  confusion of multiply setting function pointers)