This commit fixes a bug that makes the dataplane traffic of all the
emulated UEs to go through the interface of the first UE (oaitun_ue1).

The fix uses the last octet of the IP assigned to each UE to map it to its
UE_id, which is then used to get its RNTI and the correct PDCP queue.
For the emulator to work properly, the routing tables of the emulated UEs
must also be set. This patch introduces a script (setup_routes.sh) in
cmake_targets/tools that configures the routing tables. This script
must be run at the UE host machine once the UEs have successfully attached,
passing the number of emulated UEs as the argument. For example, in the
case of 16 emulated UEs, one must run "./setup_routes.sh 16".