[border-agent] enable otBorderAgentGetId API when Border Agent is enabled (#9794)

This could simplify the build configuration by omitting
`OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE`.

src/core/common/settings.cpp

@@ -106,7 +106,7 @@ void SettingsBase::SrpServerInfo::Log(Action aAction) const
 }
 #endif
 
-#if OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE
+#if OPENTHREAD_CONFIG_BORDER_AGENT_ENABLE && OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE
 void SettingsBase::BorderAgentId::Log(Action aAction) const
 {
     char         buffer[sizeof(BorderAgentId) * 2 + 1];
@@ -115,7 +115,7 @@ void SettingsBase::BorderAgentId::Log(Action aAction) const
     sw.AppendHexBytes(GetId().mId, sizeof(BorderAgentId));
     LogInfo("%s BorderAgentId {id:%s}", ActionToString(aAction), buffer);
 }
-#endif // OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE
+#endif // OPENTHREAD_CONFIG_BORDER_AGENT_ENABLE && OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE
 
 #endif // OT_SHOULD_LOG_AT(OT_LOG_LEVEL_INFO)
 
@@ -531,7 +531,7 @@ void Settings::Log(Action aAction, Error aError, Key aKey, const void *aValue)
             break;
 #endif
 
-#if OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE
+#if OPENTHREAD_CONFIG_BORDER_AGENT_ENABLE && OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE
         case kKeyBorderAgentId:
             reinterpret_cast<const BorderAgentId *>(aValue)->Log(aAction);
             break;

src/core/common/settings.hpp

@@ -767,7 +767,7 @@ public:
     } OT_TOOL_PACKED_END;
 #endif // OPENTHREAD_CONFIG_SRP_SERVER_ENABLE && OPENTHREAD_CONFIG_SRP_SERVER_PORT_SWITCH_ENABLE
 
-#if OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE
+#if OPENTHREAD_CONFIG_BORDER_AGENT_ENABLE && OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE
     /**
      * Represents the Border Agent ID.
      *
@@ -784,7 +784,7 @@ public:
          * Initializes the `BorderAgentId` object.
          *
          */
-        void Init(void) { mId = {}; }
+        void Init(void) { memset(&mId, 0, sizeof(mId)); }
 
         /**
          * Returns the Border Agent ID.
@@ -813,7 +813,7 @@ public:
 
         MeshCoP::BorderAgent::Id mId;
     } OT_TOOL_PACKED_END;
-#endif // OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE
+#endif // OPENTHREAD_CONFIG_BORDER_AGENT_ENABLE && OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE
 
 protected:
     explicit SettingsBase(Instance &aInstance)

src/core/config/border_agent.h

@@ -72,7 +72,7 @@
  *
  */
 #ifndef OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE
-#define OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE 0
+#define OPENTHREAD_CONFIG_BORDER_AGENT_ID_ENABLE 1
 #endif
 
 /**