update: always scrub final swap buffer

Ensure tmpBuffer is zeroized on the common exit path in wolfBoot_swap_and_final_erase, not only in EXT_ENCRYPTED builds.

F/CI

src/update_flash.c

@@ -542,9 +542,7 @@ static int RAMFUNCTION wolfBoot_swap_and_final_erase(int resume)
 #endif
     hal_flash_lock();
 
-#ifdef EXT_ENCRYPTED
     wolfBoot_zeroize(tmpBuffer, sizeof(tmpBuffer));
-#endif
     (void)ret;
     return 0;
 }