MBEDTLS_SSL_OUT_CONTENT_LEN macro

\def MBEDTLS_SSL_OUT_CONTENT_LEN Maximum outgoing fragment length in bytes. Uncomment to set the size of the outward TLS buffer independently of the inward buffer. It is possible to save RAM by setting a smaller outward buffer, while keeping the default inward 16384 byte buffer to conform to the TLS specification. The minimum required outward buffer size is determined by the handshake protocol's usage. Handshaking will fail if the outward buffer is too small. The specific size requirement depends on the configured ciphers and any certificate data which is sent during the handshake. For absolute minimum RAM usage, it's best to enable MBEDTLS_SSL_MAX_FRAGMENT_LENGTH and reduce MBEDTLS_SSL_MAX_CONTENT_LEN. This reduces both incoming and outgoing buffer sizes. However this is only guaranteed if the other end of the connection also supports the TLS max_fragment_len extension. Otherwise the connection may fail.