net/ethoc: add Kconfig entry for the driver

Add Kconfig entry for the driver, remove #define CONFIG_ETHOC from the
only board configuration that uses it and put it into that board's
defconfig.

Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>