zephyr: Add estimated image footer size to cache in sysbuild

Adds MCUboot's estimated overhead footer size to the application's
cache when using sysbuild, this allows that information to be
propagated to applications which can use the information to reduce
the available size for an application, preventing the MCUboot
error of image too large to swap.

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