Small improvements for release script

- Print usage if no tree-ish (tag, branch or commit) was
  specified.
- Use “gntrelease.XXXXXXXXXX” as temporary directory
  template. This makes it easier to find all of them.
- Compute MD5 checksum. This doesn't cost a lot and
  might come in handy one day.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>