Mbed TLS can be configured by writing a configuration file from scratch,
without copying mbedtls_config.h. As a consequence, all the macro
definitions in mbedtls_config.h must be optional. This was not the case for
some MBEDTLS_SSL_TLS1_3_xxx macros with numerical values related to session
tickets. Fix that.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
