Convert the SUNXI_EMAC config to Kconfig. Remove it from SYS_EXTRA_OPTIONS
from many sunxi defconfig and renamed it into SUN4I_EMAC to not confuse it
with SUN8I_EMAC.

Signed-off-by: Mylène Josserand <mylene.josserand@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>