arm: mvebu: Espressobin: Define fdt_addr and kernel_addr env aliases

Boot scripts for existing Espressobin Linux distributions, like OpenWRT,
expect that U-Boot set fdt_addr and kernel_addr envs. So add env aliases
from fdt_addr_r and kernel_addr_r values.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Andre Heider <a.heider@gmail.com>