sfxge: rename hunt nvram methods and use for Medford

Some new partitions have been added, but they shouldn't need to be
handled any differently.

Submitted by: Mark Spender <mspender at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4873