Change headers with better copyright comments

- Add SPDX License Identifier and copyright notice
- Add NOTICE.md

Minor: remove unused freertos.c

NOTICE.md

@@ -0,0 +1,15 @@
+# Third-Party Notices
+
+This project is built with open source components and includes third-party software:
+
+## 1. Original Application Code
+- **Copyright (c) 2026 Erick Ahmed**
+- **License:** [GNU General Public License v3.0](https://git.erickahmed.com/erickahmed/j1939_logger/src/branch/main/LICENSE.md)
+
+## 2. STM32F4 HAL Drivers (BSD-3-Clause)
+- **Copyright (c) 2016 STMicroelectronics**
+- **License:** [BSD 3-Clause License](https://git.erickahmed.com/erickahmed/j1939_logger/src/branch/main/Drivers/STM32F4xx_HAL_Driver/LICENSE.txt)
+
+## 3. STM32 CMSIS (Apache-2.0)
+- **Copyright (c) 2017 STMicroelectronics**
+- **License:** [Apache License 2.0](https://git.erickahmed.com/erickahmed/j1939_logger/src/branch/main/Drivers/CMSIS/LICENSE.txt)