sfxge(4): precheck and verify flash writes

Read existing flash content before writing, so the flash write can be
avoided if the existing partition content matches the new image. This
avoids unnecessary write cycles for the flash device, and may also be
faster. If the flash does need to be updated, verify the content after
writing.

Note that reading the flash content after writing but before calling
efx_nvram-rw_finish() avoids firmware bug68170, which can lead to
signed image updates failing on Medford.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18093