board: am335x: Always set eth/eth1addr environment variable

Ethernet ports might be used in the kernel even if CPSW driver
is disabled at u-boot. So always set ethaddr and eth1addr
environment variable from efuse.

Retain usbnet_devaddr as it is required for SPL USB eth boot.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>