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
* Use "example" in all example function & variable names,
ie use i2c_example_xxx instead of i2c_xxx for example functions.
Closes#198https://github.com/espressif/esp-idf/issues/198
* Mark example functions, etc. static
* Replace uses of "test" & "demo" with "example"
* Split the UART example into two
* Rename "main" example files to end with "_main.c" for disambiguation
