omap2: set the ethaddr as well as the usbethaddr env var to ensure static MAC

The kernel gets the ethernet MAC from the ethaddr variable, the omap boards for
devices with USB based eth adapters just set the usbethaddr which doesn't appear
to get passed to the kernel. The same Raspberry Pi code sets both ethaddr and
usbethaddr so lets do that so linux (tested 4.13 and 4.14) get a static rather
than a random MAC address, while not regressing users of usbethaddr.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>