scripts: Use short tag format for releases

Currently, OSv release numbers would look like this:

  $ git describe --tags --long
  v0.02-0-gc40b86b

Stop using "--long" and let "git describe" do the right thing:

  $ git describe --tags
  v0.01-298-g42d11fb
  $ git tag v0.02
  $ git describe --tags
  v0.02

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>