bootutil: loader: Add post copy hook to swap function

Currently the post copy hook is only called from the `copy_region`
function. However when another update method than `BOOT_UPGRADE_ONLY` is
selected this function is not called. This adds post copy hook to the
end of `boot_swap_image` when we know the swap is complete.

Signed-off-by: Sigvart Hovland <sigvart.hovland@nordicsemi.no>