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/dbau1550_el_defconfig b/configs/dbau1550_el_defconfig
index 3b1d74d..5b22978 100644
--- a/configs/dbau1550_el_defconfig
+++ b/configs/dbau1550_el_defconfig
@@ -2,3 +2,4 @@
 CONFIG_TARGET_DBAU1X00=y
 CONFIG_DBAU1550=y
 CONFIG_SYS_LITTLE_ENDIAN=y
+CONFIG_CMD_NET=y