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

radio/rfsimulator/CMakeLists.txt

@@ -0,0 +1,11 @@
+add_library(rfsimulator MODULE
+            simulator.c
+            apply_channelmod.c
+            ../../openair1/PHY/TOOLS/signal_energy.c
+)
+target_link_libraries(rfsimulator PRIVATE SIMU)
+target_link_libraries(rfsimulator PRIVATE asn1_nr_rrc asn1_lte_rrc)
+set_target_properties(rfsimulator PROPERTIES LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})
+
+add_executable(replay_node stored_node.c)
+target_link_libraries (replay_node minimal_lib)