sstate: Add sstate usage summary to the build

sstate: Add sstate usage summary to the build

Currently the user has no indication of how much sstate was already present
or that would be used by the build. This change adds some summary information
so that the user can see how much reuse is occurring. To fully work it
needs some extra information from a recent bitbake commit but this is
optional.

When combined with bitbake --dry-run this feature can be used to check
if sstate would be reused in a build.

[YOCTO #12749]

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