bootutil: Add active slot number and max app size to shared data

This allows the currently executing slot number to be checked by
the external function, which can be used by XIP images to know
which slot is currently being executed from to allow for correct
uploading/positioning of firmware files, and also provides the
maximum size of an upgrade that can be loaded so that applications
can reject images that are too large.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>