Move radio CMakeLists.txt to subdirectory

- move following radios: AW2S, BladeRF, IRIS, LMSSDR, RFsimulator, USRP
- move following transports: Ethernet, benetel4g, benetel5g
- for each: create independent cmake variable to switch compilation
  on/off, in order: OAI_AW2S, OAI_BLADERF, OAI_IRIS, OAI_LMSSDR,
  OAI_SIMU, OAI_USRP, OAI_ETHERNET, OAI_BENETEL4G, OAI_BENETEL5G
- Remove RF_BOARD variable, allows to compile multiple boards at once
- where applicable: remove USERSPACE/LIB in path, i.e., move files up to
  directories, as all are userspace libraries

Note that with the above, it is now mandatory to enable a radio
implementation before compiling it, e.g., for USRP:

./build_oai -w USRP
cmake .. -DOAI_USRP=ON && ninja oai_usrpdevif

executables/nr-softmodem.c

@@ -40,7 +40,7 @@
 //#undef FRAME_LENGTH_COMPLEX_SAMPLES //there are two conflicting definitions, so we better make sure we don't use it at all
 
 #include "radio/COMMON/common_lib.h"
-#include "radio/ETHERNET/USERSPACE/LIB/if_defs.h"
+#include "radio/ETHERNET/if_defs.h"
 
 //#undef FRAME_LENGTH_COMPLEX_SAMPLES //there are two conflicting definitions, so we better make sure we don't use it at all