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/ima3-mx53_defconfig b/configs/ima3-mx53_defconfig
index 0ada8a1..3190b71 100644
--- a/configs/ima3-mx53_defconfig
+++ b/configs/ima3-mx53_defconfig
@@ -1,3 +1,4 @@
 CONFIG_ARM=y
 CONFIG_TARGET_IMA3_MX53=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/esg/ima3-mx53/imximage.cfg"
+CONFIG_CMD_NET=y