OPENTHREAD_CONFIG_MESSAGE_BUFFER_SIZE macro

@def OPENTHREAD_CONFIG_MESSAGE_BUFFER_SIZE The size of a message buffer in bytes. Message buffers store pointers which have different sizes on different system. Setting message buffer size according to the CPU word length so that message buffer size will be doubled on 64bit system compared to that on 32bit system. As a result, the first message always have some bytes left for small packets. Some configuration options can increase the buffer size requirements, including OPENTHREAD_CONFIG_MLE_MAX_CHILDREN and OPENTHREAD_CONFIG_COAP_BLOCKWISE_TRANSFER_ENABLE.