NcpBase: Implement get handler for IPV6_LL_ADDR spinel property (#1826)

This commit add a new API `otThreadGetLinkLocalIp6Address()` to
get the Thread link-local address (which is is derived using
IEEE802.15.4 Extended Address as Interface Identifier).  The new
API is then used to implement the get handler for spinel property
`IPV6_LL_ADDR` in `NcpBase` class.

include/openthread/thread.h

@@ -275,6 +275,17 @@ OTAPI const uint8_t *OTCALL otThreadGetMeshLocalPrefix(otInstance *aInstance);
  */
 OTAPI otError OTCALL otThreadSetMeshLocalPrefix(otInstance *aInstance, const uint8_t *aMeshLocalPrefix);
 
+/**
+ * This function returns the Thread link-local IPv6 address.
+ *
+ * The Thread link local address is derived using IEEE802.15.4 Extended Address as Interface Identifier.
+ *
+ * @param[in]  aInstance A pointer to an OpenThread instance.
+ *
+ * @returns A pointer to Thread link-local IPv6 address.
+ */
+const otIp6Address *otThreadGetLinkLocalIp6Address(otInstance *aInstance);
+
 /**
  * Get the Thread Network Name.
  *

src/core/api/thread_api.cpp

@@ -195,6 +195,11 @@ exit:
     return error;
 }
 
+const otIp6Address *otThreadGetLinkLocalIp6Address(otInstance *aInstance)
+{
+    return &aInstance->mThreadNetif.GetMle().GetLinkLocalAddress();
+}
+
 const char *otThreadGetNetworkName(otInstance *aInstance)
 {
     return aInstance->mThreadNetif.GetMac().GetNetworkName();

src/core/thread/mle.cpp

@@ -675,6 +675,11 @@ exit:
     return error;
 }
 
+const Ip6::Address &Mle::GetLinkLocalAddress(void) const
+{
+    return mLinkLocal64.GetAddress();
+}
+
 otError Mle::UpdateLinkLocalAddress(void)
 {
     mNetif.RemoveUnicastAddress(mLinkLocal64);

src/core/thread/mle.hpp

@@ -650,6 +650,16 @@ public:
      */
     otError SetMeshLocalPrefix(const uint8_t *aPrefix);
 
+    /**
+     * This method returns a reference to the Thread link-local address.
+     *
+     * The Thread link local address is derived using IEEE802.15.4 Extended Address as Interface Identifier.
+     *
+     * @returns A reference to the Thread link local address.
+     *
+     */
+    const Ip6::Address &GetLinkLocalAddress(void) const;
+
     /**
      * This method updates the link local address.
      *

src/ncp/ncp_base.cpp

@@ -3131,10 +3131,30 @@ otError NcpBase::GetPropertyHandler_IPV6_ML_ADDR(uint8_t header, spinel_prop_key
 
 otError NcpBase::GetPropertyHandler_IPV6_LL_ADDR(uint8_t header, spinel_prop_key_t key)
 {
-    // TODO!
+    otError errorCode = OT_ERROR_NONE;
-    (void)key;
+    const otIp6Address *address = otThreadGetLinkLocalIp6Address(mInstance);
 
-    return SendLastStatus(header, SPINEL_STATUS_UNIMPLEMENTED);
+    if (address)
+    {
+        errorCode = SendPropertyUpdate(
+                        header,
+                        SPINEL_CMD_PROP_VALUE_IS,
+                        key,
+                        SPINEL_DATATYPE_IPv6ADDR_S,
+                        address
+                    );
+    }
+    else
+    {
+        errorCode = SendPropertyUpdate(
+                        header,
+                        SPINEL_CMD_PROP_VALUE_IS,
+                        key,
+                        SPINEL_DATATYPE_VOID_S
+                    );
+    }
+
+    return errorCode;
 }
 
 otError NcpBase::GetPropertyHandler_IPV6_ADDRESS_TABLE(uint8_t header, spinel_prop_key_t key)