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/iconnect_defconfig b/configs/iconnect_defconfig
index 7ff8d67..a96b810 100644
--- a/configs/iconnect_defconfig
+++ b/configs/iconnect_defconfig
@@ -1,3 +1,4 @@
 CONFIG_ARM=y
 CONFIG_KIRKWOOD=y
 CONFIG_TARGET_ICONNECT=y
+CONFIG_CMD_NET=y