bst-external 0.6

o New git-tag plugin: An extension to the BuildStream git plugin
  which allows automatic tracking of the latest tag in a branch.
o fastboot_bootimg Element - Uses artifacts generated by buildstream to produce an
  image that can be flashed to the boot partition using fastboot.
o fastboot_ext4 Element - Uses artifacts generated by buildstream to produce an
  ext4sparse image that can be flashed to userdata partitions or others using fastboot.