sfxge(4): support Medford bootcfg partition layout in common code

For Siena and Huntington, the per-port bootcfg (aka expcfg) is
stored in a dedicated 4Kbyte partition for each port.

For Medford, the per-PF bootcfg is stored in a 2Kbyte sector
within a single shared partition. Update the common code to support
the new bootcfg layout.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D8932