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/bf537-stamp_defconfig b/configs/bf537-stamp_defconfig
index 50483b1..753bbc4 100644
--- a/configs/bf537-stamp_defconfig
+++ b/configs/bf537-stamp_defconfig
@@ -1,5 +1,5 @@
 CONFIG_BLACKFIN=y
-CONFIG_NET=y
 CONFIG_TARGET_BF537_STAMP=y
+CONFIG_CMD_NET=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_CC_OPTIMIZE_LIBS_FOR_SPEED=y