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/at91sam9m10g45ek_nandflash_defconfig b/configs/at91sam9m10g45ek_nandflash_defconfig
index 844acd4..b57f2ca 100644
--- a/configs/at91sam9m10g45ek_nandflash_defconfig
+++ b/configs/at91sam9m10g45ek_nandflash_defconfig
@@ -3,3 +3,4 @@
 CONFIG_TARGET_AT91SAM9M10G45EK=y
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9M10G45,SYS_USE_NANDFLASH"
+CONFIG_CMD_NET=y