esp8266: Support dedicated REPL loop (aka pull-style).

Event-driven loop (push-style) is still supported and default (controlled
by MICROPY_REPL_EVENT_DRIVEN setting, as expected).

Dedicated loop worked even without adding ets_loop_iter(), though that
needs to be revisited later.
diff --git a/esp8266/uart.c b/esp8266/uart.c
index 9b2bfb4..bfc380a 100644
--- a/esp8266/uart.c
+++ b/esp8266/uart.c
@@ -233,6 +233,7 @@
 void mp_keyboard_interrupt(void);
 
 int interrupt_char;
+#if MICROPY_REPL_EVENT_DRIVEN
 void uart_task_handler(os_event_t *evt) {
     if (pyexec_repl_active) {
         // TODO: Just returning here isn't exactly right.
@@ -264,3 +265,4 @@
 void uart_task_init() {
     system_os_task(uart_task_handler, UART_TASK_ID, uart_evt_queue, sizeof(uart_evt_queue) / sizeof(*uart_evt_queue));
 }
+#endif