Support for installing external BSP installer packages

We would like to make possible for 3rd parties to create BSP (Board Support Package) installer packages that can be installed using the MaintenanceTool to an existing Qt SDK installation.

The BSP installation package would be single file (7z?) that is downloaded from the board vendor, but when installed acts as any other component available from the online (or offline) installer. Meaning that MaintenanceTool shows what components and versions are available in the BSP and user can select components that he wants to install.

Technically the BSP could be a regular online repo content (Updates.xml + meta.7z + content.7z) which is packaged to a single archive file (.qtbsp?). When selected to be installed, the MaintenanceTool would extract the content to a temporary directory and handle it as a Temporary Repository. The installerscript.qs in the BSP could the reuse the existing components and variables available in the regular SDK installation (e.g., @SDKToolBinary@)