esp8266: Enable inline Xtensa assembler.

With this patch, @micropython.asm_xtensa can be used on the esp8266 port.
diff --git a/esp8266/main.c b/esp8266/main.c
index 1e988ef..9883f9f 100644
--- a/esp8266/main.c
+++ b/esp8266/main.c
@@ -58,7 +58,7 @@
     MP_STATE_PORT(mp_kbd_exception) = mp_obj_new_exception(&mp_type_KeyboardInterrupt);
     MP_STATE_PORT(term_obj) = MP_OBJ_NULL;
     MP_STATE_PORT(dupterm_arr_obj) = MP_OBJ_NULL;
-    #if MICROPY_EMIT_XTENSA
+    #if MICROPY_EMIT_XTENSA || MICROPY_EMIT_INLINE_XTENSA
     extern void esp_native_code_init(void);
     esp_native_code_init();
     #endif