bootutil: swap_scratch: 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 <michal.kozikowski@nordicsemi.no>