lib/interrupt_char: Factor out typical Ctrl+C handling from esp8266 port.

Utility functions for keyboard interrupt handling, to be reused across
(baremetal) ports.
diff --git a/esp8266/Makefile b/esp8266/Makefile
index 26b5e86..9008f20 100644
--- a/esp8266/Makefile
+++ b/esp8266/Makefile
@@ -128,6 +128,7 @@
 	timeutils/timeutils.c \
 	utils/pyexec.c \
 	utils/pyhelp.c \
+	utils/interrupt_char.c \
 	fatfs/ff.c \
 	fatfs/option/ccsbcs.c \
 	)
diff --git a/esp8266/esp_mphal.c b/esp8266/esp_mphal.c
index b075523..f5e284f 100644
--- a/esp8266/esp_mphal.c
+++ b/esp8266/esp_mphal.c
@@ -128,14 +128,6 @@
     mp_hal_delay_us(delay * 1000);
 }
 
-void mp_hal_set_interrupt_char(int c) {
-    if (c != -1) {
-        mp_obj_exception_clear_traceback(MP_STATE_PORT(mp_kbd_exception));
-    }
-    extern int interrupt_char;
-    interrupt_char = c;
-}
-
 void ets_event_poll(void) {
     ets_loop_iter();
     if (MP_STATE_VM(mp_pending_exception) != NULL) {
@@ -180,7 +172,7 @@
         mp_buffer_info_t bufinfo;
         mp_get_buffer_raise(MP_STATE_PORT(dupterm_arr_obj), &bufinfo, MP_BUFFER_READ);
         nlr_pop();
-        if (*(byte*)bufinfo.buf == interrupt_char) {
+        if (*(byte*)bufinfo.buf == mp_interrupt_char) {
             mp_keyboard_interrupt();
             return -2;
         }
diff --git a/esp8266/esp_mphal.h b/esp8266/esp_mphal.h
index fa52ae5..7a71c0f 100644
--- a/esp8266/esp_mphal.h
+++ b/esp8266/esp_mphal.h
@@ -28,10 +28,10 @@
 #define _INCLUDED_MPHAL_H_
 
 #include "py/ringbuf.h"
+#include "lib/utils/interrupt_char.h"
 #include "xtirq.h"
 
 void mp_keyboard_interrupt(void);
-extern int interrupt_char;
 
 struct _mp_print_t;
 // Structure for UART-only output via mp_printf()
diff --git a/esp8266/main.c b/esp8266/main.c
index 71dfeb2..22abb22 100644
--- a/esp8266/main.c
+++ b/esp8266/main.c
@@ -141,10 +141,6 @@
 }
 MP_DEFINE_CONST_FUN_OBJ_KW(mp_builtin_open_obj, 1, mp_builtin_open);
 
-void mp_keyboard_interrupt(void) {
-    MP_STATE_VM(mp_pending_exception) = MP_STATE_PORT(mp_kbd_exception);
-}
-
 void nlr_jump_fail(void *val) {
     printf("NLR jump failed\n");
     for (;;) {
diff --git a/esp8266/uart.c b/esp8266/uart.c
index d724331..001a9c6 100644
--- a/esp8266/uart.c
+++ b/esp8266/uart.c
@@ -39,8 +39,6 @@
 void soft_reset(void);
 void mp_keyboard_interrupt(void);
 
-int interrupt_char;
-
 /******************************************************************************
  * FunctionName : uart_config
  * Description  : Internal used function
@@ -172,7 +170,7 @@
 
         while (READ_PERI_REG(UART_STATUS(uart_no)) & (UART_RXFIFO_CNT << UART_RXFIFO_CNT_S)) {
             uint8 RcvChar = READ_PERI_REG(UART_FIFO(uart_no)) & 0xff;
-            if (RcvChar == interrupt_char) {
+            if (RcvChar == mp_interrupt_char) {
                 mp_keyboard_interrupt();
             } else {
                 ringbuf_put(&input_buf, RcvChar);