Remove restore method for local operational dataset. (#1958)

With the local operational dataset being retrieved from non-volatile storage
on demand, the restore method is now no longer necessary.

This PR also fixes a bug in initializing the in-memory dataset representation
when no operational dataset exists in non-volatile storage.

src/core/meshcop/dataset_local.cpp

@@ -76,9 +76,8 @@ otError DatasetLocal::Get(Dataset &aDataset)
     otError error;
 
     aDataset.mLength = sizeof(aDataset.mTlvs);
-
     error = otPlatSettingsGet(GetInstance(), GetSettingsKey(), 0, aDataset.mTlvs, &aDataset.mLength);
-    SuccessOrExit(error);
+    VerifyOrExit(error == OT_ERROR_NONE, aDataset.mLength = 0);
 
     delayTimer = static_cast<DelayTimerTlv *>(aDataset.Get(Tlv::kDelayTimer));
     VerifyOrExit(delayTimer);
@@ -107,16 +106,15 @@ otError DatasetLocal::Get(otOperationalDataset &aDataset) const
     const Tlv *cur;
     const Tlv *end;
 
-    dataset.mLength = sizeof(dataset.mTlvs);
+    memset(&aDataset, 0, sizeof(aDataset));
 
+    dataset.mLength = sizeof(dataset.mTlvs);
     error = otPlatSettingsGet(GetInstance(), GetSettingsKey(), 0, dataset.mTlvs, &dataset.mLength);
     SuccessOrExit(error);
 
     cur = reinterpret_cast<const Tlv *>(dataset.mTlvs);
     end = reinterpret_cast<const Tlv *>(dataset.mTlvs + dataset.mLength);
 
-    memset(&aDataset, 0, sizeof(aDataset));
-
     while (cur < end)
     {
         switch (cur->GetType())
@@ -404,21 +402,6 @@ exit:
     return error;
 }
 
-otError DatasetLocal::Restore(void)
-{
-    Dataset dataset(mType);
-    otError error;
-
-    dataset.mLength = sizeof(dataset.mTlvs);
-    error = otPlatSettingsGet(GetInstance(), GetSettingsKey(), 0, dataset.mTlvs, &dataset.mLength);
-    SuccessOrExit(error);
-
-    mUpdateTime = Timer::GetNow();
-
-exit:
-    return error;
-}
-
 uint16_t DatasetLocal::GetSettingsKey(void) const
 {
     uint16_t rval;

src/core/meshcop/dataset_manager.cpp

@@ -204,13 +204,7 @@ otError DatasetManager::ApplyConfiguration(void)
 
 otError DatasetManager::Restore(void)
 {
-    otError error;
-
-    SuccessOrExit(error = mLocal.Restore());
-    mLocal.Get(mNetwork);
-
-exit:
-    return error;
+    return mLocal.Get(mNetwork);
 }
 
 void DatasetManager::Clear(void)