FYI - A potentially easy way to reduce ROM Flash usage by 10672 Bytes when using MQTT-TLS.
in mbedtls config.h, comment out the following two lines:
#define MBEDTLS_SHA1_C
...
#define MBEDTLS_SHA512_C
MQTT-TLS seems to only use SHA256, so removing the other two hash libraries saves a ton of ROM flash space. @hirotakaster, might be worth adding this config change to your next release. There are some static function instances that are created with this define, so they don’t get compiler-optimized away, seemingly.