Refactor routines that read swap status data

When doing a swap some of the information required by the process
is temporarily written to the status area; previously each data
writing routines tried to find the correct area by looking for the
magic and assuming that data is located there (because everything
fits a single sector). This moves the code to find the magic into
a separate routine that is now used by others.

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