bladeRF: have it functional with libbladerf 2.0

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.