McuSupport: Find default FreeRTOS dir inside pre-installed board SDK

Qt for MCUs 1.3 ships the board/MCU SDKs, and also adds the subdir to
the FreeRTOS component inside the board/MCU SDKs as "boardSdkSubDir" to
the .json files (see: UL-2760).

Task: Qt Creator needs to lookup that "boardSdkSubDir" value and use it
to construct a default path for the FreeRTOS path.

Task-number: QTCREATORBUG-24300
Change-Id: Ie3c8186b76443d5fe3640226ea61aa8b14779d54
Reviewed-by: Rainer Keller <Rainer.Keller@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/mcusupport/mcusupportoptions.cpp

@@ -95,6 +95,11 @@ QString McuPackage::label() const
     return m_label;
 }
 
+QString McuPackage::defaultPath() const
+{
+    return m_defaultPath;
+}
+
 QString McuPackage::detectionPath() const
 {
     return m_detectionPath;