env: Rename env_driver_lookup_default and env_get_default_location

The env_driver_lookup_default and env_get_default_location functions are
about to get refactored to support loading from multiple environment.

The name is therefore not really well suited anymore. Drop the default
part to be a bit more relevant.

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>