logging: Add demultiplexer for log_frontend_stmesp

Add module which demultiplexes stream of log message which are
encoded into STPv2 stream by log_frontend_stmesp. There are 3 types of
messages that can be present in the stream:
- Log message (starts with DMTS32, followed by Dx and closed by FLAG)
- Tracepoint (FLAGTS or DMTS32)
- STM HW event (DMTS8 on 0x80 master and 0x00 channel)

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