runqueue: Add output for -S option for listing the changepoints compared with an sstate cache

Its useful to understand where the delta starts against an existing sstate cache
for a given target. Adding this to the output of the -S option seems like a
natural fit.

We use the hashvalidate function to figure this out and assume it can find siginfo
files for more than just the setscene tasks.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>