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/mx51evk_defconfig b/configs/mx51evk_defconfig
index 28cf798..e93e81f 100644
--- a/configs/mx51evk_defconfig
+++ b/configs/mx51evk_defconfig
@@ -22,6 +22,7 @@
 CONFIG_CMD_FAT=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_FSL_ESDHC=y
+CONFIG_MII=y
 CONFIG_SPI=y
 CONFIG_MXC_SPI=y
 CONFIG_USB=y