usb: net: migrate CONFIG_USB_HOST_ETHER to Kconfig

CONFIG_USB_HOST_ETHER is the framework that the drivers are dependent on
USB_HOST_ETHER. Use this as a menu and move the existing LAN75XX and
LAN78XX options under new menu. Finally update the defconfigs that need
CONFIG_USB_HOST_ETHER.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/mach-imx/mx6/Kconfig b/arch/arm/mach-imx/mx6/Kconfig
index a1aa36b..2b0cefb 100644
--- a/arch/arm/mach-imx/mx6/Kconfig
+++ b/arch/arm/mach-imx/mx6/Kconfig
@@ -307,6 +307,7 @@
 
 config TARGET_NITROGEN6X
 	bool "nitrogen6x"
+	imply USB_HOST_ETHER
 
 config TARGET_OPOS6ULDEV
 	bool "Armadeus OPOS6ULDev board"