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/Kconfig b/arch/Kconfig
index 3fc3c57..c2b4070 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -115,6 +115,7 @@
 	imply CMD_PCI
 	imply CMD_SF_TEST
 	imply CMD_ZBOOT
+	imply USB_HOST_ETHER
 
 config XTENSA
 	bool "Xtensa architecture"