esp8266/esp_mphal: Add tentative change to mp_hal_stdin_rx_chr() to wait IRQ.

Instead of busy-looping waiting for UART input. Not enabled by default,
needs more testing.

esp8266/esp_mphal.c

@@ -60,7 +60,14 @@ int mp_hal_stdin_rx_chr(void) {
         if (c != -1) {
             return c;
         }
+        #if 0
+        // Idles CPU but need more testing before enabling
+        if (!ets_loop_iter()) {
+            asm("waiti 0");
+        }
+        #else
         mp_hal_delay_us(1);
+        #endif
     }
 }