env: migrate env_get_default to call env_get_default_into

Since both functions share a similar goal and env_get_default_into can
do what env_get_default wants to do with specific arguments, let's make
env_get_default call env_get_default_into so as to avoid code
duplication.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>