rockchip: board: lion-rk3368: reduce env-size default to 8KiB

We want to have the same configuration defaults for the RK3368-uQ7
as for the RK3399-Q7: this change reduces the default env-size to
8KiB to ensure that it does not overlap the boot-payload on SD/MMC
configurations.

References: commit fe529e65 ("rockchip: rk3399-puma: reduce env size to 8kiB")
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>