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/P1021RDB-PC_SPIFLASH_defconfig b/configs/P1021RDB-PC_SPIFLASH_defconfig
index 7c8ad06..e3790e0 100644
--- a/configs/P1021RDB-PC_SPIFLASH_defconfig
+++ b/configs/P1021RDB-PC_SPIFLASH_defconfig
@@ -4,3 +4,4 @@
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="P1021RDB,SPIFLASH"
 CONFIG_CMD_SETEXPR=y
+CONFIG_CMD_NET=y