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/cmd/Kconfig b/cmd/Kconfig
index bd90946..d5abcfd 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1074,6 +1074,7 @@
 menuconfig CMD_NET
 	bool "Network commands"
 	default y
+	imply NETDEVICES
 
 if CMD_NET