bugfix(uart): patten detect function

bugfix(uart): patten detect function

requirement from github(https://github.com/espressif/esp-idf/issues/805): to provide the position in the buffer of the pattern detected.

requirement from AT application: in AT app, when no hardware flow control is enabled, in some situation the rx buffer might be full, and the terminator “+++” might be lost, we can use pattern detect interrupt to avoid missing the terminator. When pattern detect interrupt happens, it will not send a data event at the same time.

1. Add API to get position of detected pattern in rx buffer
2. Modify UART event example
3. Add comments for uart_flush, add alias API uart_flush_input to clear the rx buffer
4. Modify the way rx_buffered_len is calculated