include: configs: Update env for selecting right dtb

Now that single defconfig shall be used for booting J721S2 EVM and
AM68 SK, the default device tree will not work for selecting dtb for
kernel. Update the findfdt env to select right dtb based on
board_name env variable.

Signed-off-by: Sinthu Raja <sinthu.raja@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>