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/Cubietruck_defconfig b/configs/Cubietruck_defconfig
index e1b4495..2acc3ad 100644
--- a/configs/Cubietruck_defconfig
+++ b/configs/Cubietruck_defconfig
@@ -8,4 +8,5 @@
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="AXP209_POWER,SUNXI_GMAC,RGMII,AHCI,SATAPWR=SUNXI_GPH(12),USB_EHCI"
 CONFIG_CMD_SETEXPR=y
+CONFIG_CMD_NET=y
 CONFIG_ETH_DESIGNWARE=y