Use a new specialized ChildInfo structure for children information persistent storage (#1572)

Create a new specialized ChildInfo structure, which will be only used for children information storage

Also update otPlatSettingsGet() to be consistent with its documentation:
* return the actual length of the setting, but not the actual length it read to aValue

examples/platforms/utils/settings.cpp

@@ -378,7 +378,7 @@ ThreadError otPlatSettingsGet(otInstance *aInstance, uint16_t aKey, int aIndex,
                         utilsFlashRead(address + sizeof(struct settingsBlock), aValue, readLength);
                     }
 
-                    valueLength = readLength;
+                    valueLength = block.length;
                     error = kThreadError_None;
                 }