cmd_sf: add "update" subcommand to do smart SPI flash update

This adds a new SPI flash command which only rewrites blocks if the contents
need to change. This can speed up SPI flash programming when much of the
data is unchanged from what is already there.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>