logging: Refactor LOG_MODULE_REGISTER and LOG_MODULE_DECLARE macros

Changed LOG_MODULE_REGISTER and LOG_MODULE_DECLARE macros to take log
level as optional parameter. LOG_MODULE_DECLARE can now also be used
in static inline functions in headers. Added LOG_LEVEL_SET macro
which is used when instance logging API is used to indicate maximal
log level compiled into the file.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>