Draft: Refactor CMakeLists.txt

Refactors CMakeLists.txt to make it more readable, and to bring us closer to allow

mkdir build && cd build && cmake .. -GNinja && ninja

(i.e., compiling all targets).

Specifically, this MR

• Removes some unused CMake options
• Removes packages that are checked for but that are not used (openpgm, xpm, libcrypto)
• TODO check that those packages are not installed by default: I think they are not, but not sure; reviewer, please check
• Correctly check for dependencies (i.e., USRP has a UHD dependency, but until now, we did not check this, so add it)
  • Corollary: these targets need to be enabled now via cmake; build_oai does this automatically. If e.g., OAi has been build without UHD, doing ./build_oai -w USRP will build USRP (this did not work before)
• Reduce the amount of include_directories(): it is discouraged by cmake to use it, as it includes directories for all targets. For some libraries (openssl, libconfig, ...), we now use target_include_directories()
• Fix minimal_lib and rftest
• shlib loader is built as a separate OBJECT library to prevent multiple compilations
• Move all radio libs build instructions into separate CMakeLists.txt under radio/ to simplify the root CMakeLists.txt
• it seems the AW2S library has a dependency on the ethernet library, but it is currently possible to build one without the other. should be fixed (edit: fixed)
• Simplify build_oai and tools/build_helper

More detailed information is in the commit messages.

The output of cmake is now cleaner, and it lists found libraries, like so:

-- Checking for module 'libcrypt'
--   Found libcrypt, version 4.4.10
-- Checking for module 'libconfig'
--   Found libconfig, version 1.5
-- Checking for module 'openssl'
--   Found openssl, version 1.1.1f
-- Checking for module 'blas'
--   Found blas, version 3.10.3
-- Checking for module 'lapacke'
--   Found lapacke, version 3.9.0
-- Checking for module 'cblas'
--   No package 'cblas' found

Bonus: for one machine, the compilation time drops from 5min to 4min 50sec (around 3% faster)