tools: enable multi-year copyright year checking

license-check logic is extended to parse and understand
multiple copyright years other than the current year.

Signed-off-by: Yefeng Lin <yefeng.lin@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>

Change-Id: If9f6de4af096302cb2f45664f8140cb5a98e77c8