boot: bootutil: loader: Fix issue with stuck revert

Fixes an issue (only for swap using move, unsure if it affects
other modes) whereby a device can get into a state that there is
a valid primary image, no secondary image but the primary image
flag is still set to revert, which then makes loading a firmware
update from the application impossible due to it assuming that a
revert will happen upon device reboot

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>