[mle] add mHasRestored flag (#5228)

This commit introduces the mHasRestored flag and a common
HasRestored() method as devices may decide to introduce some
jitter/randomization to avoid the burst of traffic in cases a bunch of
devices nearby resets/reboots together.

src/core/thread/mle.cpp

@@ -86,6 +86,7 @@ Mle::Mle(Instance &aInstance)
     , mDataRequestAttempts(0)
     , mDataRequestState(kDataRequestNone)
     , mAddressRegistrationMode(kAppendAllAddresses)
+    , mHasRestored(false)
     , mParentLinkMargin(0)
     , mParentIsSingleton(false)
     , mReceivedResponseFromParent(false)
@@ -439,6 +440,9 @@ otError Mle::Restore(void)
     }
 #endif
 
+    // Sucessfully restored the network information from non-volatile settings after boot.
+    mHasRestored = true;
+
 exit:
     return error;
 }

src/core/thread/mle.hpp

@@ -915,6 +915,16 @@ public:
      */
     otError GetLocatorAddress(Ip6::Address &aAddress, uint16_t aLocator) const;
 
+    /**
+     * This method indicates whether or not the device has restored the network information from
+     * non-volatile settings after boot.
+     *
+     * @retval true  Sucessfully restored the network information.
+     * @retval false No valid network information was found.
+     *
+     */
+    bool HasRestored(void) const { return mHasRestored; }
+
 protected:
     /**
      * States during attach (when searching for a parent).
@@ -1734,6 +1744,7 @@ private:
 
     AddressRegistrationMode mAddressRegistrationMode;
 
+    bool       mHasRestored;
     uint8_t    mParentLinkMargin;
     bool       mParentIsSingleton;
     bool       mReceivedResponseFromParent;