Real-YModem implemented a flexible YModem support. It use callback-based
structure to let the user application to deal with the data. It contains
3 examples:
1. echo.c: write the data recieved on YModem to an other device
2. null.c: discard the YModem data
3. tofile.c: write the data to the file system
Currently, it does not support batch file transmission.
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