Introduce env support for sata device.
1. Implement write_env/read_env/env_relocate_spec/saveenv/sata_get_env_dev
2. If want to enable this feature, define CONFIG_ENV_IS_IN_SATA, and
   define CONFIG_SYS_SATA_ENV_DEV or implement your own sata_get_ev_dev.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Stuart Longland <stuartl@vrt.com.au>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>

This is an enhancement that permits storing the environment file on an
EXT4 partition such as the root filesystem.  It is based on the existing
FAT environment file code.

Now that they are in their own directory, we can remove this prefix.
This makes it easier to find a file since the prefix does not get in the
way.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: Przemyslaw Marczak <p.marczak@samsung.com>