drivers: Add "from micropython import const" when const is used.

Following best-practice use of the const feature, to make it compatible
with Python.

drivers/display/ssd1306.py

 # MicroPython SSD1306 OLED driver, I2C and SPI interfaces
 
+from micropython import const
 import time
 import framebuf
 

drivers/nrf24l01/nrf24l01.py

 """NRF24L01 driver for Micro Python
 """
 
+from micropython import const
 import pyb
 
 # nRF24L01+ registers

drivers/sdcard/sdcard.py

@@ -21,6 +21,7 @@ Example usage on ESP8266:
 
 """
 
+from micropython import const
 import time
 
 

esp8266/modules/ds18x20.py

 # DS18x20 temperature sensor driver for MicroPython.
 # MIT license; Copyright (c) 2016 Damien P. George
 
+from micropython import const
+
 _CONVERT = const(0x44)
 _RD_SCRATCH = const(0xbe)
 _WR_SCRATCH = const(0x4e)

esp8266/modules/onewire.py

 # 1-Wire driver for MicroPython on ESP8266
 # MIT license; Copyright (c) 2016 Damien P. George
 
+from micropython import const
 import _onewire as _ow
 
 class OneWireError(Exception):

stmhal/boards/STM32F4DISC/staccel.py

@@ -14,6 +14,7 @@ See:
     STM32Cube_FW_F4_V1.1.0/Projects/STM32F4-Discovery/Demonstrations/Src/main.c
 """
 
+from micropython import const
 from pyb import Pin
 from pyb import SPI