This commit adds a new log output model for NCP to use a newly added
spinel stream log property. `OPENTHREAD_CONFIG_LOG_OUTPUT_NCP_SPINEL`
can be used to be select the new log output model.
This commit defines a new spinel property `SPINEL_PROP_STREAM_LOG`
which provides streaming of formatted log string from NCP along with
an optional metadata structure. OpenThread log level and log region
are included as part of the metadata.
This commit also updates the `toranj` configuration header file to
use the new log output model.
Oleksandr Grytsov