sfxge(4): add buffer editing functions to boot config

Functions to process the DHCP option list format used by the expansion
ROM config buffers, to support extracting and updating of individual
options.
The initial use case is the driver presenting the global and per-PF
options as separate items, with the driver implementing the
synchronization of global options across the configuration buffers
for all PFs.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18254