zephyr: Use flash_area_flatten in boot_set_next

Switch from using flash_area_erase to flash_area_flatten.
The later function uses write on devices that do not have erase by
design, to scramble data.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

boot/bootutil/src/bootutil_public.c

@@ -548,7 +548,11 @@ boot_set_next(const struct flash_area *fa, bool active, bool confirm)
             /* The image slot is corrupt.  There is no way to recover, so erase the
              * slot to allow future upgrades.
              */
+#if !defined(CONFIG_MCUBOOT) && defined(__ZEPHYR__)
+            flash_area_flatten(fa, 0, flash_area_get_size(fa));
+#else
             flash_area_erase(fa, 0, flash_area_get_size(fa));
+#endif
             rc = BOOT_EBADIMAGE;
         }
         break;