board: ls2080a: Add "mcinitcmd" env for MC & DPL deployment

Environment variable mcinitcmd is defined to initiate MC and DPL
deployment from the location where it is stored (NOR, NAND, SD, SATA,
USB) during booting. If this variable is not defined then macro
MC_BOOT_ENV_VAR will be null and MC will not be booted and DPL will
not be applied during U-boot booting.

Signed-off-by: Pratiyush Mohan Srivastava <pratiyush.srivastava@nxp.com>
Signed-off-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>