src: log.h: Remove leftover of log_output_* functions
src: log: Document and expose buffering details of log module to other modules

This exposes some details about the buffering nature of the log module to the
outside world and documents it in a way, that it should become clear why and
how the buffering works. Furthermore it adapts callback functions that were
flushing the UART buffer explicitely beforehand.
src: dcf77.c: Replace leftovers of LOG_DCF with proper log messages