make logging configurable during compilation and runtime

    Compile-time:
      * A new meson option 'log_level' is introduced which has all the values from the
        Loglevel enum
      * The 'OFF' option defines EMPER_LOG_OFF which causes the LOG* macros to be empty
        and the Logger<>::log function to return immediately
      * The default 'automatic' option sets 'log_level' to 'Error' for release builds
        and to 'ALL' otherwise

    Runtime:
      * The global variable emper::log_level defined in Debug.hpp controls the logging output
      * It is initialized with the meson option 'log_level' but can be changed during runtime