am335x_evm: enable support for booting via USB

This adds necessary config options and a new build target,
am335x_evm_usbspl, to enable usb booting and fixes board_eth_init()
function to take into account that we may have USB ether support in SPL
now.  This uses the same MAC for both cpsw and USB, in order to match
ROM behavior.

The usbspl build target does not contain UART SPL, CPSW SPL or extra
environment settings, so that we may fit within our binary size
constraint.

Signed-off-by: Ilya Yanok <ilya.yanok@cogentembedded.com>
Signed-off-by: Tom Rini <trini@ti.com>