Dataset storage (#866)

* Store active dataset and pending dataset in non-volatile memory.

  * Retrieve active dataset and pending dataset from non-volatile memory after each reboot.

  * Leader will generate the active dataset if it is not initialized.

  * Clear pending dataset after it replaces active dataset.

  * Create different pseudo flash files for posix nodes.

include/platform/settings.h

@@ -36,6 +36,7 @@
 #define OT_PLATFORM_SETTINGS_H 1
 
 #include <stdint.h>
+#include <openthread-instance.h>
 
 #ifdef __cplusplus
 extern "C" {