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.
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