- lock with mutexes
- don't start ENB_APP task, but Agent directly and wait in case of
  reconfiguration

- {restart,stop}_L1L2() in lte-softmodem.c
- add function start_phy_rrc() in enb_app.{c,h}, accessible from outside
- will be used to restart PHY and RRC when lte-softmodem is restarted

use only one FlexRAN wait function

- when exiting task, the task is set as "not configured"
- this permits to start a task again

*Fix the issue with the PDCP UE ID/RNTI management

protobuf-c does not compile anymore.

Let's handle this a bit better.

We now install protobuf and protobuf-c only for the
flexran agent. That is, if you want to use the flexran
agent, you need to install protobuf/protobuf-c and
you do it this way:

  ./build_oai -I -a

(you add -a)

Other targets don't need protobuf nor protobuf-c, so
it's not installed by the -I command of build_oai,
unless you pass -a with -I.

Also, we now use protobuf 3.3.0, not 2.6.1. The code
has been adapted, a quick test seems to indicate that
the system works, but it has not been intensively tested.

Merge branch 'feature-68-enb-agent' of https://gitlab.eurecom.fr/oai/openairinterface5g into feature-68-enb-agent