feat(slave_node): Implement initial Modbus RTU server

- Add a basic Modbus RTU server implementation based on Zephyr samples.
- Configure usart1 for Modbus via a board overlay.
- The server initializes and runs, but polling with mbpoll results in a timeout.
- This commit captures a functional but non-working state for further debugging.

software/apps/slave_node/prj.conf

@@ -1,6 +1,6 @@
 # Enable Console and printk for logging
 CONFIG_CONSOLE=y
-CONFIG_PRINTK=y
+CONFIG_LOG=y
 
 # Disable UART console
 CONFIG_UART_CONSOLE=n
@@ -8,3 +8,8 @@ CONFIG_UART_CONSOLE=n
 # Enable RTT console
 CONFIG_RTT_CONSOLE=y
 CONFIG_USE_SEGGER_RTT=y
+
+# Config modbus
+CONFIG_UART_INTERRUPT_DRIVEN=y
+CONFIG_MODBUS=y
+CONFIG_MODBUS_ROLE_SERVER=y