am335x_evm: Clarify when we build board_eth_init

If we build this function in cases where we would be discarding it
anyhow we still end up with maybe unused warnings.  Rather than litter
the function with __maybe_unused, just spell out when to build it.

Signed-off-by: Tom Rini <trini@ti.com>
am335x_evm: Drop CONFIG_SPL_ETH_SUPPORT from default build

On the boards this target supports this option is either non possible
without hardware mods (Beaglebone White/Black) or not supported due to
board design.  Drop this and regain some space.

Signed-off-by: Tom Rini <trini@ti.com>