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/d2net_v2_defconfig b/configs/d2net_v2_defconfig
index e54dcf4..a28032e 100644
--- a/configs/d2net_v2_defconfig
+++ b/configs/d2net_v2_defconfig
@@ -2,3 +2,4 @@
 CONFIG_KIRKWOOD=y
 CONFIG_TARGET_NET2BIG_V2=y
 CONFIG_SYS_EXTRA_OPTIONS="D2NET_V2"
+CONFIG_CMD_NET=y