[BOOTSCRIPT][U-BOOT] Add fdt overlay support
Added running of a variable coming from internal environment provided by SECO. So it is supported by the internal BSP only. If fdt_overlay_load2ram is set, it will be executed.
This change allows the output of seco_confing command to be executed.
Signed-off-by: Davide Cardillo davide.cardillo@seco.com