Convert CONFIG_MII et al to Kconfig

This converts the following to Kconfig:
   CONFIG_MII
   CONFIG_DRIVER_TI_EMAC

Signed-off-by: Adam Ford <aford173@gmail.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/configs/am335x_sl50_defconfig b/configs/am335x_sl50_defconfig
index b4dfe01..399fdd6 100644
--- a/configs/am335x_sl50_defconfig
+++ b/configs/am335x_sl50_defconfig
@@ -42,6 +42,7 @@
 CONFIG_BOOTCOUNT_LIMIT=y
 CONFIG_MMC_OMAP_HS=y
 CONFIG_DRIVER_TI_CPSW=y
+CONFIG_MII=y
 CONFIG_SYS_NS16550=y
 CONFIG_SPI=y
 CONFIG_OMAP3_SPI=y