Fix setting confirm flag

It was impossible to set confirm flag due to incorrect comparison.
In this point of time state->image_ok is already decoded and contains
one of following values:

If BOOT_FLAG_UNSET then we are ok to set a confirm flag.

Signed-off-by: Łukasz Rymanowski <lukasz.rymanowski@codecoup.pl>

boot/bootutil/src/bootutil_misc.c

@@ -574,7 +574,7 @@ boot_set_confirmed(void)
         goto done;
     }
 
-    if (state_slot0.image_ok != erased_val) {
+    if (state_slot0.image_ok != BOOT_FLAG_UNSET) {
         /* Already confirmed. */
         goto done;
     }