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/harmony_defconfig b/configs/harmony_defconfig
index b2c0afc..aa556a1 100644
--- a/configs/harmony_defconfig
+++ b/configs/harmony_defconfig
@@ -3,3 +3,4 @@
 CONFIG_TEGRA20=y
 CONFIG_TARGET_HARMONY=y
 CONFIG_DEFAULT_DEVICE_TREE="tegra20-harmony"
+CONFIG_CMD_NET=y