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/common/Kconfig b/common/Kconfig
index 2e3b8c0..a2167f0 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -266,6 +266,7 @@
 
 config CMD_NET
 	bool "bootp, tftpboot"
+        select NET
 	help
 	  Network commands.
 	  bootp - boot image via network using BOOTP/TFTP protocol