Fix multiple PDUSessions after review, now DRBs are configured sequentially, no matter what PDUSession id we receive

- Add E2 agent that leverages FlexRIC e2ap library
- Add documentation in openair2/E2AP/README.md
- Currently, statistics exported are random

Instead of using ninja/make directly, there is a cmake command option to
trigger the build using whatever build tool has been configured. Use it;
if there is a mismatch, there will also be a clearer cmake error.

Remove build_oai's --build-coverity: we don't use it, and it can
trivially be added if required (probably, most users would run it by
hand anyway).

Add an error if the compilations function is not called with the right
noumber of arguments.

If it was one, build_oai without parameters would attempt to build the
ethernet library. However, this is unexpected, and will lead to an error
(as it is by default off in CMake). Set it to off, and build it where
required.

- 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

- libcrypto is not used
- we use libcrypt, so correctly check for it, and link to the
  corresponding targets

Use target_include_directories() only on relevant targets.
Remove from some targets that don't use the config module.