env: Migrate CONFIG_ENV_AES to Kconfig and deprecate

The underlying implementation for ENV_AES has security complications and
is not recommended for use.  Please see CVE-2017-3225 and CVE-2017-3226
for more details.  Mark this as deprecated now and delete this in the
medium term if no one comes forward to re-work the support.

Signed-off-by: Tom Rini <trini@konsulko.com>