rk3399: Enable SF distro bootcmd

Enable SPI flash(SF) distro boot command in rk3399.

This distro boot will read the boot script at specific
location at the flash and start sourcing the same.

Included the SF device at the last of the target devices
list since all the rest of the devices on the list have
more possibility to boot the distribution due to the
size of the SPI flash is concern.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>