Generate sysroots of freedesktop-sdk

Here I use the tar_element plugin to generate a gzipped tarball of both the sdk and platform. I also add targets to build and export tarballs in the Makefile, and add a job in the CI to build tarballs on every tagged pipeline. I restricted this to tags as building a tarball of the platform locally took half an hour, so building a load of tarballs on every branch would do our already-too-long CI no favours.

Additionally I have bumped the version of bst-external, to get at the plugin.

I don't know how we would plan to release the tarballs generated, and versioning them in the CI might take a little more work.

Closes #114 (closed)