logging: log_frontend: Add optional API for common messages

Extend frontend API with optional set of functions which can
be used when simplified log message handling is enabled. If this
mode is enabled then there are dedicated macros for processing the
most common messages (string + 0-2 word arguments). Using this API
can speed up the processing of messages that are the most common.

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