Add MQTT Broker with TLS support for STM32H753ZI

[dgarske]

Requires: wolfSSL/wolfMQTT#463

Summary

Ports the MQTT broker implementation from STM32H563 to STM32H753ZI, enabling a fully functional MQTT broker with TLS 1.3 on port 8883.

Changes

wolfIP (src/port/stm32h753/)

• Added mqtt_broker.c and mqtt_broker.h - broker wrapper for bare-metal integration
• Updated Makefile - added ENABLE_MQTT_BROKER build option with wolfMQTT sources
• Updated user_settings.h - configured wolfMQTT broker with optimized limits for 512KB RAM
  • BROKER_MAX_CLIENTS=3 (reduced from 4 on H563)
  • WOLFMQTT_BROKER_CUSTOM_NET for application-managed TLS context
• Updated config.h - increased MAX_TCPSOCKETS from 8 to 12, adjusted buffers for broker
• Updated main.c - integrated broker initialization and polling

Memory Usage

• Flash: 222KB (fits in 1MB)
• RAM: 490KB (fits in 512KB)
• Broker supports 3 concurrent TLS clients

Testing

# Build and flash
make ENABLE_MQTT_BROKER=1 CC=arm-none-eabi-gcc \
     WOLFSSL_ROOT=../../../../wolfssl \
     WOLFMQTT_ROOT=../../../../wolfmqtt
st-flash write app.bin 0x08000000

# Test broker
mosquitto_pub -h 10.0.4.201 -p 8883 --insecure -t test -m "Hello"
mosquitto_sub -h 10.0.4.201 -p 8883 --insecure -t test/# -v

Testing output

=== Clock Config Debug ===
Running at 64MHz HSI...
Configuring HSE + PLL (400MHz)...
Clock config OK! Now at 400MHz SYSCLK, 200MHz HCLK
RNG init: OK val=0xF1A664F3


=== wolfIP STM32H753ZI Echo Server ===
Initializing wolfIP stack...
Initializing Ethernet MAC...
  PHY link: UP, PHY addr: 0x00000000
Waiting for DHCP...
DHCP configuration received.
IP Address: 10.0.4.201
Subnet Mask: 10.0.4.1
Gateway: 10.0.4.1
DNS Server: 10.0.4.1
DHCP configuration received:
  IP: 10.0.4.201
  Mask: 10.0.4.1
  GW: 10.0.4.1
Creating TCP echo server on port 7...
Initializing MQTT broker on port 8883 (TLS)...
MQTT Broker: Initializing
Entering main loop. Ready for connections!
  TCP Echo: port 7
MQTT Broker: TLS initialized (TLS 1.3, ECC P-256)
MQTT Broker: Running on port 8883 (TLS)