[tlv] Change method AppendTlv to accept uint16_t.

Change `AppendTlv` to accept `uint16_t` and in case of TLV's
longer than 254 uses ExtendedTlv.

src/core/common/tlvs.cpp

@@ -269,16 +269,24 @@ template Error Tlv::AppendUintTlv<uint8_t>(Message &aMessage, uint8_t aType, uin
 template Error Tlv::AppendUintTlv<uint16_t>(Message &aMessage, uint8_t aType, uint16_t aValue);
 template Error Tlv::AppendUintTlv<uint32_t>(Message &aMessage, uint8_t aType, uint32_t aValue);
 
-Error Tlv::AppendTlv(Message &aMessage, uint8_t aType, const void *aValue, uint8_t aLength)
+Error Tlv::AppendTlv(Message &aMessage, uint8_t aType, const void *aValue, uint16_t aLength)
 {
-    Error error = kErrorNone;
-    Tlv   tlv;
-
-    OT_ASSERT(aLength <= Tlv::kBaseTlvMaxLength);
+    Error       error = kErrorNone;
+    ExtendedTlv extTlv;
+    Tlv         tlv;
 
-    tlv.SetType(aType);
-    tlv.SetLength(aLength);
-    SuccessOrExit(error = aMessage.Append(tlv));
+    if (aLength > kBaseTlvMaxLength)
+    {
+        extTlv.SetType(aType);
+        extTlv.SetLength(aLength);
+        SuccessOrExit(error = aMessage.Append(extTlv));
+    }
+    else
+    {
+        tlv.SetType(aType);
+        tlv.SetLength(static_cast<uint8_t>(aLength));
+        SuccessOrExit(error = aMessage.Append(tlv));
+    }
 
     VerifyOrExit(aLength > 0);
     error = aMessage.AppendBytes(aValue, aLength);

src/core/common/tlvs.hpp

@@ -574,6 +574,9 @@ class Tlv
     /**
      * Appends a TLV with a given type and value to a message.
      *
+     * If the TLV length is longer than maximum base TLV size defined by `kBaseTlvMaxLength` then
+     * appends extended TLV.
+     *
      * On success this method grows the message by the size of the TLV.
      *
      * @param[in]  aMessage      The message to append to.
@@ -585,7 +588,7 @@ class Tlv
      * @retval kErrorNoBufs   Insufficient available buffers to grow the message.
      *
      */
-    static Error AppendTlv(Message &aMessage, uint8_t aType, const void *aValue, uint8_t aLength);
+    static Error AppendTlv(Message &aMessage, uint8_t aType, const void *aValue, uint16_t aLength);
 
     /**
      * Appends a TLV with a given type and value to a message.