rockchip: puma-rk3399: load environment from same medium as one used to load U-Boot proper

Chances are when one boots U-Boot proper from a given storage medium,
they want the same medium to be used to load and store the environment.

This basically allows to have completely separate U-Boot (TPL/SPL/U-Boot
proper/environment) per storage medium which is convenient when working
with recovery from SD-Card as one would just need to insert a properly
configured SD-Card into the device to have access to their whole debug
setup.

No fallback mechanism is provided as to not dirty other storage medium
environment by mistake. However, since arch_env_get_location() is called
by env_init() which is part of the pre-relocation process, a valid,
non-ENVL_UNKNOWN, value shall be returned otherwise the relocation fails
with the following message:
initcall sequence 00000000002866c0 failed at call 0000000000256b34 (err=-19)

This valid, non-ENVL_UNKNOWN, value is ENVL_NOWHERE which requires to
always select CONFIG_ENV_IS_NOWHERE otherwise this work-around does not
work.

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>