changes:
- ue mcc/mnc 208.93
- use correct key/opc for user
- change addresses in conf file for them to be easier to understand
  (maybe)

With those changes, running:
   sudo ./run_enb_ue_virt_s1
in cmake_targets/tools should work out of the box

The user still has to configure correct IP addresses in
targets/PROJECTS/GENERIC-LTE-EPC/CONF/enb.band7.generic.oaisim.local_mme.conf

We supposed oaisim (enb+ue) machine to be on IP address 10.0.1.1
and EPC (hss, mme, spgw) machine to be on IP address 10.0.1.2.

- print current working directory
- print git version (if available)
- print security keys
  this last one is a security breach, but as of today
  it's not a problem

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.

Keys/IDs have been inadvertently changed.

We'll need to to do it in a separate thread/process.
To be done. In the meantime, put it back. It impacts
oaisim.

The compilation line was:
    ./build_oai --eNB -w USRP

The file openairinterface5g/cmake_targets/log/lte-softmodem.Rel10.txt
has been checked and all LOG_X (and 'msg') warnings have been fixed.

This reverts commit d31634c3.

Laurent Thomas had a problem on one machine with the build_oai
way of checking for nettle.

The problem with the alternative solution of including nettle/bignum.h
is that it is very unclear.

The problem with nettle is that the file nettle/config.h does not
exist for version 2. It was introduced in version 3.

We want to support both versions, but there is an API incompatibility.
So we need an #if #else mechanism.

The file nettle/bignum.h is present in both versions 2 and 3 and it
includes nettle/version.h in the version 3.

So by including this file, we can check for the existence of
NETTLE_VERSION_MAJOR (that comes from nettle/config.h) in the
code.

But as you can see, the reasoning is way too complex.

So it's better to keep the check in cmake_targets/CMakeLists.txt.

As long as we support version 2 this will be the way to go.

It is possible to force a given version in specific non-generic
customized environments.

Those functions modify a global char array (a string).

Let's pass a buffer to those functions, so that it's
thread safe. The caller has been modified, with hopefully
a buffer big enough (still bigger than what was there before,
so should not break more than it did).

- fix OPc key
- clear EHPLMN_LIST, UE does not start the RA procedure when set
  to be fixed

Guess what happens when we return from the function...

The case instance == INSTANCE_DEFAULT should normally not happen,
but let's be a bit verbose if it does.

oaisim is not functional with several UEs.
This fixes one issue. (It's still not functional for other reasons.)

It was not setting emm_data->eplmn.plmn[X], only emm_data->eplmn.n_plmns,
leading to the UE to dig for PLMN 00000.

(It's not the end of the story, seems like the PLMN selection
does not work, the UE only tries one from what I've seen, but
I didn't dig much, may be wrong.)

Adapt calling sites too.

When data comes from the user, it is good to check that
what we read is correct and warn the user if it's not.

Also included some source files not related to this branch.

	modified:   openair1/PHY/LTE_ESTIMATION/lte_dl_bf_channel_estimation.c
	modified:   openair1/PHY/LTE_TRANSPORT/pilots_ue_spec.c
	modified:   openair1/PHY/MODULATION/beamforming.c
	modified:   openair1/PHY/TOOLS/twiddle18432.h
	modified:   openair1/SIMULATION/LTE_PHY/dlsim_tm7.c
	modified:   openair3/NAS/TOOLS/nvram.c
	modified:   openair3/NAS/TOOLS/usim.c
	modified:   openair3/NAS/UE/user_defs.h

That's not the end of the story though.

I am not sure if it's the right way to solve the
problem, so I keep the definitions here and
put them inside #if 0 #endif

In openairinterface5g many definitions in many layers of UE or eNB NB_MAX.
We created a file called openairinterface5g_limits.h
located at openairinterface5g_dir/targets/COMMON/ declaring the number max
of UE and eNB to be used in ALL layers depending on launching conditions
(LARGE_SCALE for example).