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/am335x_boneblack_vboot_defconfig b/configs/am335x_boneblack_vboot_defconfig
index e805c9d..a8584e9 100644
--- a/configs/am335x_boneblack_vboot_defconfig
+++ b/configs/am335x_boneblack_vboot_defconfig
@@ -8,4 +8,5 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_FIT_SIGNATURE=y
 CONFIG_SYS_EXTRA_OPTIONS="EMMC_BOOT,ENABLE_VBOOT"
+CONFIG_CMD_NET=y
 CONFIG_OF_CONTROL=y