Update OpenMote board header file to define UPDATE_CCA.

As per the cc2538dk and ReMote boards. This enables the cc2538's
bootloader backdoor, to allow flashing via the serial interface even
when a valid image is already present.

Backdoor enable is set to LOW, PORT A, PIN 6 (ON/SLEEP on the OpenBase),
for consistency with the default OpenMote firmware.

boards/openmote-cc2538/include/board.h

@@ -71,6 +71,19 @@
 #define XTIMER_ISR_BACKOFF  (40)
 /** @} */
 
+/**
+ * @name Flash Customer Configuration Area (CCA) parameters
+ * @{
+ */
+#ifndef UPDATE_CCA
+#define UPDATE_CCA                (1)
+#endif
+
+#define CCA_BACKDOOR_ENABLE       (1)
+#define CCA_BACKDOOR_PORT_A_PIN   (6) /**< ON/SLEEP Pin */
+#define CCA_BACKDOOR_ACTIVE_LEVEL (0) /**< Active low */
+/** @} */
+
 /**
  * @brief Initialize board specific hardware, including clock, LEDs and std-IO
  */