OPENTHREAD_CONFIG_LOG_OUTPUT macro

@def OPENTHREAD_CONFIG_LOG_OUTPUT Selects if, and where the LOG output goes to. There are several options available - @sa OPENTHREAD_CONFIG_LOG_OUTPUT_NONE - @sa OPENTHREAD_CONFIG_LOG_OUTPUT_DEBUG_UART - @sa OPENTHREAD_CONFIG_LOG_OUTPUT_APP - @sa OPENTHREAD_CONFIG_LOG_OUTPUT_PLATFORM_DEFINED - and others Note: 1) Because the default is: OPENTHREAD_CONFIG_LOG_OUTPUT_PLATFORM_DEFINED The platform is expected to provide at least a stub for `otPlatLog()`. 2) This is effectively an ENUM so it can be if/else/endif at compile time.