bootutil/loader: Allow not working secondary image device

Let's distinguish a case when secondary image device has a malfunction
from case when primary has a such.
It might be still possible to boot the primary image.

This make sens especially if the secondary image resides in an
external flash which might be damaged while SoC is still working.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>