Fix review issues
Fix review issues

- Use git options to remove last commit from list to verify.
- Check each line of a commit for a "Signed-off-by" line.
- Exit with error in the event of no commits in PR!

Signed-off-by: Fabio Utzig <66676b1ceaf93296e098708c09494361103aa635@apache.org>

ATmobica/mcuboot,ATmobica/mcuboot,utzig/mcuboot,utzig/mcuboot,utzig/mcuboot,tamban01/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,utzig/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,tamban01/mcuboot