sfxge: erase nvram partitions in chunks equal to their erase size

The erase size is reported by the nvram info command.

Submitted by: Paul Fox <pfox at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4386