esp8266/esp_mphal: Implement libc's errno.

Using __errno() function, and redirect it to use mp_stream_errno from
stream module. This is pre-requisite for integrating with 3rd-party libs,
like BerkeleyDB.

esp8266/esp_mphal.c

@@ -259,3 +259,8 @@ int ets_esf_free_bufs(int idx) {
     }
     return cnt;
 }
+
+extern int mp_stream_errno;
+int *__errno() {
+    return &mp_stream_errno;
+}