For certain tasks, we need additional information in build stamp file
other than the task name and file name. stamp-extra-info is introduced as
a task flag which is appended to the stamp file name.
[Code simplifcations/tweaks from Richard]
Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>