net: openthread: Change OT net log to display current role in text

If CONFIG_NET_LOG=y is set, OpenThread will output the current OT
role whenever the state changes.

To simplify understanding of the log output, this change replaces
the numerical role ID with the text name of the role. This also
required a change to a documentation file to replace an instance
of a numerical ID.

NOTE: This is potentially a breaking change should anyone be using
test scripts that monitor the OpenThread state changes and look
for the numerical ID. This does not seem to be the case for the
Zephyr tests, however.

Signed-off-by: Chris Pearson <ctpearson@gmail.com>
2 files changed