env_sf: support embedded environments

If both CONFIG_ENV_SECT_SIZE and CONFIG_ENV_SIZE are defined, and the sect
size is larger than the env size, then it means the env is embedded in a
block.  So we have to save/restore the part of the sector which is not the
environment.  Previously, saving the environment in SPI flash in this
setup would probably brick the board as the rest of the sector tends to
contain actual U-Boot data/code.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>