freemodbus: fix port contains lgpl licensed files

Initial version of freemodbus master port files have been added to ESP-IDF based on https://github.com/armink/FreeModbus_Slave-Master-RTT-STM32.
The overall repository license, at the time of adding these files, has been BSD 3-clause. However at that time, several port files carried LGPL license headers. As the author of these files confirmed in https://github.com/armink/FreeModbus_Slave-Master-RTT-STM32/issues/61#issuecomment-977828450, this wasn't intentional. ESP-IDF version of modbus master port has been rewritten to target FreeRTOS instead of RT-Thread, but the license headers remained from the original version. This commit corrects this, replacing the license of these files with BSD 3-clause.

docs/en/COPYRIGHT.rst

@@ -73,6 +73,8 @@ These third party libraries can be included into the application (firmware) prod
 
 * :component_file:`UBSAN runtime <esp_system/ubsan.c>` — Copyright (c) 2016, Linaro Limited and Jiří Zárevúcky, licensed under the BSD 2-clause license.
 
+* `freemodbus`_ Copyright (c) 2006-2013 Christian Walter, Armink and licensed under the BSD license.
+
 Build Tools
 -----------
 
@@ -186,3 +188,4 @@ Copyright (C) 2011, ChaN, all right reserved.
 .. _sphinx_rtd_theme: https://github.com/readthedocs/sphinx_rtd_theme
 .. _cryptoauthlib: https://github.com/MicrochipTech/cryptoauthlib
 .. _qrcode: https://github.com/nayuki/QR-Code-generator
+.. _freemodbus: https://github.com/armink/FreeModbus_Slave-Master-RTT-STM32