VELAPLATFO-4612
There are three test.
1. Write a long default string into the serial port.
2. Read the default string given by the last test from the serial port.
3. Manually execute the `test_content_ gen.py` script and write the text into the serial device to test the burst capability or run this script and specify device path to automatically test.
Signed-off-by: xinbingnan <xinbingnan@xiaomi.com>
