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/mx6dlarm2_defconfig b/configs/mx6dlarm2_defconfig
index 48216f6..f4130fb 100644
--- a/configs/mx6dlarm2_defconfig
+++ b/configs/mx6dlarm2_defconfig
@@ -25,4 +25,5 @@
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
+CONFIG_USB_HOST_ETHER=y
 CONFIG_OF_LIBFDT=y