Enable ROCKCHIP_SPL_BACK_TO_BROM for rock2 board

Rock2 has been tested with back to brom feature. The tricky part is that
with this feature the default environment is inside u-boot, and it's
defined for every rk3288 board independetly. So I just changed it for
rock2 here if ROCKCHIP_SPL_BACK_TO_BROM.

Solve by moving environment after u-boot before 1M boundary

Signed-off-by: Sandy Patterson <apatterson@sightlogix.com>
Acked-by: Simon Glass <sjg@chromium.org>