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/BSC9131RDB_NAND_SYSCLK100_defconfig b/configs/BSC9131RDB_NAND_SYSCLK100_defconfig
index a2ac062..4ebe4a4 100644
--- a/configs/BSC9131RDB_NAND_SYSCLK100_defconfig
+++ b/configs/BSC9131RDB_NAND_SYSCLK100_defconfig
@@ -4,3 +4,4 @@
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="BSC9131RDB,NAND,SYS_CLK_100"
 CONFIG_CMD_SETEXPR=y
+CONFIG_CMD_NET=y