bootutil: fix swap with move reset issue

Fix a swap corruption which occurs on the swap move algorithm when a
reset happens exactly at the point after the last move up, and its
status update. On restart the image headers should be read at the 2nd
sector of the primary slot, but due to lacking initialization it is
read on the first sector, and then fails. This error was masked on the
simulator because of the use of a global variable, which retained its
value on a "reset simulation".

Fixes #1588

Signed-off-by: Fabio Utzig <utzig@apache.org>