net: Move the CMD_NET config to defconfigs

This also selects CONFIG_NET for any CONFIG_CMD_NET board.

Remove the imx default for CONFIG_NET.

This moves the config that was defined by 60296a8 (commands: add more
command entries in Kconfig).

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/configs/spear320_nand_defconfig b/configs/spear320_nand_defconfig
index 347b3de..1023111 100644
--- a/configs/spear320_nand_defconfig
+++ b/configs/spear320_nand_defconfig
@@ -1,6 +1,6 @@
 CONFIG_ARM=y
 CONFIG_TARGET_SPEAR320=y
-CONFIG_NET=y
 CONFIG_NETDEVICES=y
 CONFIG_SYS_EXTRA_OPTIONS="spear320,nand"
+CONFIG_CMD_NET=y
 CONFIG_ETH_DESIGNWARE=y