esp8266: Enable sys.{stdin,stdout,stderr}.buffer for raw serial access.

esp8266/mpconfigport.h

@@ -50,6 +50,7 @@
 #define MICROPY_PY_SYS_MAXSIZE      (1)
 #define MICROPY_PY_SYS_EXIT         (1)
 #define MICROPY_PY_SYS_STDFILES     (1)
+#define MICROPY_PY_SYS_STDIO_BUFFER (1)
 #define MICROPY_PY_UERRNO           (1)
 #define MICROPY_PY_UBINASCII        (1)
 #define MICROPY_PY_UCTYPES          (1)