feat: Establish functional multi-app structure

This commit captures a working multi-app build where the board definition is located in the 'software' directory and explicitly included by the slave_node application. This serves as a stable baseline.

software/boards/iten/bluepill_f103rb/bluepill_f103rb.dts

@@ -0,0 +1,106 @@
+/*
+ * Copyright (c) 2025 Eduard Iten
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/dts-v1/;
+#include <st/f1/stm32f1.dtsi>
+#include <st/f1/stm32f103Xb.dtsi>
+#include <st/f1/stm32f103r(8-b)tx-pinctrl.dtsi>
+#include <zephyr/dt-bindings/input/input-event-codes.h>
+
+/ {
+	model = "Blue-Pill STM32F103RB";
+	compatible = "iten,bluepill-f103rb";
+
+	chosen {
+		zephyr,console = &usart1;
+		zephyr,shell-uart = &usart1;
+		zephyr,sram = &sram0;
+		zephyr,flash = &flash0;
+	};
+
+	leds {
+		compatible = "gpio-leds";
+		led_status: led_status {
+			gpios = <&gpioc 13 GPIO_ACTIVE_LOW>;
+			label = "User LED";
+		};
+	};
+
+	aliases {
+		led0 = &led_status;
+		watchdog0 = &iwdg;
+	};
+};
+
+
+&clk_lsi {
+	status = "okay";
+};
+
+&clk_hse {
+	clock-frequency = <DT_FREQ_M(8)>;
+	status = "okay";
+};
+
+&pll {
+	mul = <9>;
+	clocks = <&clk_hse>;
+	status = "okay";
+};
+
+&rcc {
+	clocks = <&pll>;
+	clock-frequency = <DT_FREQ_M(72)>;
+	ahb-prescaler = <1>;
+	apb1-prescaler = <2>;
+	apb2-prescaler = <1>;
+	adc-prescaler = <6>;
+};
+
+
+&usart1 {
+	pinctrl-0 = <&usart1_tx_pa9 &usart1_rx_pa10>;
+	pinctrl-names = "default";
+	current-speed = <115200>;
+	status = "okay";
+};
+
+&iwdg {
+	status = "okay";
+};
+
+
+&clk_lsi {
+	status = "okay";
+};
+
+&clk_hse {
+	clock-frequency = <DT_FREQ_M(8)>;
+	status = "okay";
+};
+
+&pll {
+	mul = <9>;
+	clocks = <&clk_hse>;
+	status = "okay";
+};
+
+&rcc {
+	clocks = <&pll>;
+	clock-frequency = <DT_FREQ_M(72)>;
+	ahb-prescaler = <1>;
+	apb1-prescaler = <2>;
+	apb2-prescaler = <1>;
+	adc-prescaler = <6>;
+};
+
+&exti {
+	status = "okay";
+};
+
+&dma1 {
+	status = "okay";
+};