shared: Add support for non-native architectures to dissect_image()

shared: Add support for non-native architectures to dissect_image()

To allow dissecting images of architectures other than the native
(or secondary) one, we add a third designator 'OTHER' to represent
architectures other than the native or secondary one.

If no partitions of the native or secondary arch are available, we
check if a root partition of any other arch is available and use that
instead if we found one.