[nrf fromtree] bootutil: Fix scratch scrambling

This commit fixes the scrambling of the scratch area after
persisting a trailer to the primary slot. Previous use of
boot_erase_region() didn't guarantee that the scratch area is
scrambled, as it only erases memory for devices that selects
CONFIG_MCUBOOT_STORAGE_WITH_ERASE.

Signed-off-by: Michal Kozikowski <[email protected]>
(cherry picked from commit 800868a)

Author: nordic-mik7

boot/bootutil/src/swap_scratch.c

@@ -926,7 +926,7 @@ boot_swap_sectors(int idx, uint32_t sz, struct boot_loader_state *state,
             * happens then the scratch which is partially erased would be wrote back to the
             * primary slot, causing a corrupt unbootable image
             */
-            rc = boot_erase_region(fap_scratch, 0, flash_area_get_size(fap_scratch), true);
+            rc = boot_scramble_region(fap_scratch, 0, flash_area_get_size(fap_scratch), true);
             assert(rc == 0);
         }
     }