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/ap_sh4a_4a_defconfig b/configs/ap_sh4a_4a_defconfig
index f76329b..16c1132 100644
--- a/configs/ap_sh4a_4a_defconfig
+++ b/configs/ap_sh4a_4a_defconfig
@@ -1,2 +1,3 @@
 CONFIG_SH=y
 CONFIG_TARGET_AP_SH4A_4A=y
+CONFIG_CMD_NET=y