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/gwventana_defconfig b/configs/gwventana_defconfig
index 898b3c5..5cfe983 100644
--- a/configs/gwventana_defconfig
+++ b/configs/gwventana_defconfig
@@ -6,3 +6,4 @@
 CONFIG_SPL_STACK_R_ADDR=0x18000000
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,MX6QDL"
 CONFIG_CMD_SETEXPR=y
+CONFIG_CMD_NET=y