Step 2 complete: MCUboot integration with single-slot configuration

- Created sysbuild configuration for MCUboot bootloader
- Added device tree overlays for correct partition layout (32KB MCUboot, 96KB app)
- Fixed MCUboot partition addressing to use boot_partition instead of slot0_partition
- MCUboot successfully boots and chains to application
- Application runs with shell and custom reset command
- Disabled signature validation for testing purposes

software/apps/firmware_node/sysbuild/firmware_node.overlay

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2021 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&flash0 {
+        partitions {
+                compatible = "fixed-partitions";
+                #address-cells = <1>;
+                #size-cells = <1>;
+
+                boot_partition: partition@0 {
+                        label = "mcuboot";
+                        reg = <0x00000000 0x00008000>; /* 32 KB */
+                        read-only;
+                };
+
+                slot0_partition: partition@8000 {
+                        label = "image-0";
+                        reg = <0x00008000 0x00018000>; /* 96 KB */
+                };
+        };
+};
+
+/ {
+	chosen {
+		zephyr,code-partition = &slot0_partition;
+	};
+};