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 buand 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