logging: log_output: Move flushing and writing to the header

Move log_output_flush and log_output_write (renamed internal
buffer_write() function) to the header as inline functions.
Those function are used by log_output_dict.c and there are cases
when log_output.c is not compiled in.

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