This commit introduces several enhancements in X2AP and RRC to deal
with ENDC. The main goal is to deal with UE's on/off and with various
errors.

The following work is included:

    - DC prep and DC overall X2 timers
    - X2 messages for UE release and DC timers' timeout

Open problems:

    - what to do when the UE sends an scg Failure? This commit removes
      the UE. And this removal may not be correctly done
    - UE removal in the gNB's PHY layer (a next commit is coming,
      but it is hackish and will need a rework)

In the configuration file, to enable measurements, add:

    enable_measurement_reports = "yes";

Note that if x2 is enabled then the option 'enable_measurement_reports'
is not taken into account and the measurements are enabled.

This commit introduces X2AP timers (t_reloc_prep, tx2_reloc_overall).
You need to set the values in the configuration file.

X2AP can be enabled or disabled in the configuration file too (disabled
by default).

Some deadcode was removed.

Not much has been changed in the driver (that used libbladerf 1.0).
Some bugs have been fixed. Documentation has been added. A configuration
file has been added too, put in the directory ./configuration/bladeRF.

To use a bladeRF device, see documentation in targets/ARCH/BLADERF/README.

Only the BladeRF x40 has been tested.

Performance is not too bad at 5MHz, a bit worse at 10MHz and on the test
machine I used I face realtime problems at 20MHz when I push downlink
throughput.

So, there is still some work to do to have a good support of bladeRF.