OPENTHREAD_CONFIG_ENABLE_DEBUG_UART macro

@def OPENTHREAD_CONFIG_ENABLE_DEBUG_UART Enable the "Debug Uart" platform feature. In the embedded world, the CLI application uses a UART as a console and the NCP application can be configured to use either a UART or a SPI type device to transfer data to the host. The Debug UART is or requires a second uart on the platform. The Debug Uart has two uses: Use #1 - for random 'debug printf' type messages a developer may need Use #2 (selected via DEBUG_LOG_OUTPUT) is a log output. See #include for more details