cmd: nvedit: Get rid of the env lookup

The nvedit command is the only user of env_driver_lookup_default outside of
the environment code itself, and it uses it only to print the environment
it's about to save to during env save.

As we're about to rework the environment to be able to handle multiple
environment sources, we might not have an idea of what environment backend
is going to be used before trying (and possibly failing for some).

Therefore, it makes sense to remove that message and move it to the
env_save function itself. As a side effect, we also can get rid of the call
to env_driver_lookup_default that is also about to get refactored.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>