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/at91sam9n12ek_mmc_defconfig b/configs/at91sam9n12ek_mmc_defconfig
index d9cf7d1..463e23c 100644
--- a/configs/at91sam9n12ek_mmc_defconfig
+++ b/configs/at91sam9n12ek_mmc_defconfig
@@ -2,3 +2,4 @@
 CONFIG_ARCH_AT91=y
 CONFIG_TARGET_AT91SAM9N12EK=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9N12,SYS_USE_MMC"
+CONFIG_CMD_NET=y