am335x_evm: Never set CONFIG_EXTRA_ENV_SETTINGS in SPL

Because of our support for network-based SPL, we don't discard all of
the environment related functions.  We however never make use of the
default CONFIG_EXTRA_ENV_SETTINGS items and as this variable grows, it
brings us closer to (or with some toolchains, over) our SPL size limit.
Never set this in the case of SPL.

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