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/tb100_defconfig b/configs/tb100_defconfig
index c8a71bf..c753197 100644
--- a/configs/tb100_defconfig
+++ b/configs/tb100_defconfig
@@ -2,11 +2,11 @@
 CONFIG_ARC_CACHE_LINE_SHIFT=5
 CONFIG_TARGET_TB100=y
 CONFIG_SYS_CLK_FREQ=500000000
-CONFIG_NET=y
 CONFIG_NETDEVICES=y
 CONFIG_DM_SERIAL=y
 CONFIG_SYS_TEXT_BASE=0x84000000
 CONFIG_DEFAULT_DEVICE_TREE="abilis_tb100"
+CONFIG_CMD_NET=y
 CONFIG_OF_CONTROL=y
 CONFIG_OF_EMBED=y
 CONFIG_DM=y