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/riotboard_defconfig b/configs/riotboard_defconfig
index a53ee94..104b3e6 100644
--- a/configs/riotboard_defconfig
+++ b/configs/riotboard_defconfig
@@ -2,5 +2,6 @@
 CONFIG_TARGET_EMBESTMX6BOARDS=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/boundary/nitrogen6x/nitrogen6s1g.cfg,MX6S,DDR_MB=1024,ENV_IS_IN_MMC"
 CONFIG_CMD_SETEXPR=y
+CONFIG_CMD_NET=y
 CONFIG_DM=y
 CONFIG_DM_THERMAL=y