This commit adds support for the dynamic log level selection. Config
option `OPENTHREAD_CONFIG_ENABLE_DYNAMIC_LOG_LEVEL` can be used to
enable/disable this feature (default setting is disabled).
Note that the `OPENTHREAD_CONFIG_LOG_LEVEL` specifies the log level at
compile/build time. The dynamic log level feature (if enabled) only
allows decreasing log level from the compile time setting.
This commit also adds `SPINEL_PROP_DEBUG_NCP_LOG_LEVEL` as a new
spinel property to control the log level. The set/get handlers for
this property along with its documentation are also added.
This commit modifies some of the log related macros/methods so
to harmonize them all to require an `otInstance` as their first
argument.
Jonathan Hui