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

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