fix(slave_node): Stabilize Modbus RTU communication

The Modbus server was previously unstable, leading to intermittent CRC errors when polled by a master. This was caused by the main thread exiting after initialization, which created timing and race condition issues for the interrupt-driven Modbus stack.

This fix ensures the main thread continues to run in a low-power sleep loop (). This provides a stable context for the Modbus server, resolving the CRC errors and ensuring reliable communication.

software/apps/slave_node/boards/bluepill_f103rb.overlay

@@ -1,3 +1,17 @@
+/ {
+    chosen {
+        zephyr,console = &rtt;
+        zephyr,shell-uart = &rtt; // If using shell
+    };
+
+    rtt: rtt {
+        compatible = "segger,rtt-uart";
+        #address-cells = <1>;
+        #size-cells = <0>;
+        label = "RTT";
+        status = "okay";
+    };
+};
 &usart1 {
 	modbus0 {
 		compatible = "zephyr,modbus-serial";