boot: Add IMAGE_F_ROM_FIXED flag support to mcuboot

The IMAGE_F_ROM_FIXED, now supported by imgtool and mcumgr, allows to
use image_header.ih_load_addr to store fixed ROM address information.
The address is provided for Direct-XIP mcuboot to be able to verify
if it can boot an image in given slot, by checking if the address
the image has been built for corresponds to slot address.
This flag is not required to be set within image, in which case
the check will not happen, but is highly desired as it allows mcuboot
to detect images that have been mismatched with slot.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>