bootutil: fix scratch status offset calculation

Update `boot_status_off` to take into consideration that scratch only
has to store status for one swap of sectors, thus requiring much less
flash space than what is required in the primary slot.

This commits does a bit of refactoring on the functions that return
parts of trailer size to make the code clearer.

Signed-off-by: Fabio Utzig <utzig@apache.org>