board/ls2080ardb: Update board env based on SoC

As per current implementation, default value of board env is
based on board filename i.e ls2080ardb.

With distro support changes, this env is used to decide upon
kernel dtb which is different for other SoCs (ls2088a, ls2081a)
combination supported with this board.

Add support to modify board env at runtime based on SoC type

Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>