Add 'factoryreset' cli (#892)

* add 'factoryreset' cli to delete all stored settings and then reset

* use 'factoryreset' in THCI reset API to ensure that there is no pre-settings in DUT before running any test

include/openthread.h

@@ -1657,6 +1657,13 @@ void otSetAssignLinkQuality(otInstance *aInstance, const uint8_t *aExtAddr, uint
  */
 void otPlatformReset(otInstance *aInstance);
 
+/**
+ * This method deletes all the settings stored on non-volatile memory, and then triggers platform reset.
+ *
+ * @param[in]  aInstance A pointer to an OpenThread instance.
+ */
+void otFactoryReset(otInstance *aInstance);
+
 /**
  * Get the ROUTER_DOWNGRADE_THRESHOLD parameter used in the Router role.
  *