Function ets_printf

pub unsafe extern "C" fn ets_printf(fmt: *const u8, ...) -> i32

@brief Printf the strings to uart or other devices, similar with printf, simple than printf. Can not print float point data format, or longlong data format. So we maybe only use this in ROM.

@param const char *fmt : See printf.

@param … : See printf.

@return int : the length printed to the output device.