rockchip: ringneck-px30: enforce ENV_IS_NOWHERE with Kconfig select

rockchip: ringneck-px30: enforce ENV_IS_NOWHERE with Kconfig select

Instead of letting the compiler error out if CONFIG_ENV_IS_NOWHERE is
not selected by the user, let's just enforce it when the user builds for
Ringneck PX30 so that no check needs to be performed by the compiler and
the configuration is always valid.

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