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/board.yml

@@ -0,0 +1,8 @@
+# Copyright (c) 2025 Eduard Iten
+# SPDX-License-Identifier: Apache-2.0
+
+board:
+  name: bluepill_f103rb
+  vendor: iten
+  socs:
+  - name: stm32f103xb