Add basic IDLE feature

[TG9541]

The IDLE feature shall be controlled by the variable 'IDLE (see target/'IDLE), and it can do mundane tasks like flashing an LED when the console waits for the next character to arrive. At 9600 baud, and for most use cases, the IDLE routine should be faster than 1ms (except if ?RX gets replaced by something interrupt driven, and buffered).