board/lx2160ardb: Add distro boot support

Add u-boot enviroments to support distro boot which scan
boot.scr from external storage devices
(e.g. SD/USB/SCSI disk) and execute autoboot script

Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>
Reviewed-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>