rk3399: Add BOOTENV_SF command

Add missing BOOTENV_SF command in rk3399 config.

Fix it.

Fixes: f263b860 ("rk3399: Enable SF distro bootcmd")
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reported-by: Suniel Mahesh <sunil@amarulasolutions.com>
Tested-by: Suniel Mahesh <sunil@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>