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/configs/colibri_imx6_defconfig b/configs/colibri_imx6_defconfig
index 0c52de5..7d1e709 100644
--- a/configs/colibri_imx6_defconfig
+++ b/configs/colibri_imx6_defconfig
@@ -59,6 +59,7 @@
 CONFIG_G_DNL_MANUFACTURER="Toradex"
 CONFIG_G_DNL_VENDOR_NUM=0x1b67
 CONFIG_G_DNL_PRODUCT_NUM=0x4000
+CONFIG_USB_HOST_ETHER=y
 CONFIG_FAT_WRITE=y
 CONFIG_OF_LIBFDT=y
 CONFIG_OF_LIBFDT_OVERLAY=y