rk3399_common: setup fdtoverlay_addr_r value

fdtoverlay (pxe_utils) require define fdtoverlay_addr_r env variable
for example sunxi-common.h meson64.h already have it.

Signed-off-by: Artem Lapkin <art@khadas.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>