loader: add checking of reset address

In a multi image context it is possible for a user to upload an image
to the wrong secondary slot. As the same key is used for both images
MCUboot will overwrite image 0 with a variant of image 1.

If direct overwrite is enabled it is not trivial to recover from this.

To mitigate this issue we introduce a check of the reset address within
the vector table.

If the reset address in the new image is not contained within the
primary slot the image is deemed incorrect and is deleted from the
secondary slot

Signed-off-by: Håkon Øye Amundsen <haakon.amundsen@nordicsemi.no>