sfxge(4): add functions to efx_bootcfg supporting whole partition access

Expose expcfg partition layout discovery and validating buffer copy
routines. Needed for whole-partition expcfg operations.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D8970