- 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

Optimisation of rx_pdcch : save some time by reading one symbol pdcch first and read symbol 2 and 3 if needed
Optimisation of nb of candidate when searching DCI

Using "system" to call "git log" is too error-prone.
In the test setup, lte-softmodem was blocked because
"git log" uses a pager ("less") and the pager was
waiting for the user to press enter.

We could pass "--no-pager" to "git log" but then
what if someone runs the softmodem out of the source
tree?

cmake defines PACKAGE_VERSION, we can use that instead,
as is done in openair-cn.

see https://gitlab.eurecom.fr/oai/openairinterface5g/issues/227

When the UE connects to the eNodeB and receives its IP address from the
network, it calls system() to set it in the linux kernel world. This call
is not done in a realtime thread, but in the NAS, which uses its own thread,
independent of the realtime processing.

In some situations this totally disrupts realtime processing.

It is difficult to know precisely why that happens, but it seems that calling
fork(), as system() does, in a multi-threaded program is not a good idea. (So
say several people on the internet.) It is not clear why the softmodem is
impacted, but it seems that fork() is really what triggers the disruption.
Several tests lead to that conclusion.

To fix the problem, we create a child background process very early in main()
(before anything else basically). Then instead of calling system(), the main
process sends the string to the background process. The background process
gets the string, passes it to system() and reports the success/failure back
to the main process.

This solution involves a lot of system calls, but calling system() in the
first place is not cheap either. As long as no realtime thread uses this
mechanism, things should be fine. Time will tell.

Bug reported by Bruno Mongazon from Nokia.

"Without this line, the config file parameter is not properly stored and
the FH will always work with compression."

For whatever reason, lte-softmodem.c was totally messed up...

We took the version from the branch develop and added
abstraction_flag, which is needed in openair2/LAYER2/MAC/main.c
(this is not clean, previous commit added a comment for this).
This has to be cleaned at some point.

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.

see issue #43

tested with both EXMIMO and USRP. performance better than in master branch.

debugging information in lte-enb, addition of external reference option in lte-softmodem and usrp_lib.cpp