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/rpi_3_defconfig b/configs/rpi_3_defconfig
index 31c4130..bfab5fc 100644
--- a/configs/rpi_3_defconfig
+++ b/configs/rpi_3_defconfig
@@ -29,6 +29,7 @@
 CONFIG_USB_STORAGE=y
 CONFIG_USB_KEYBOARD=y
 CONFIG_SYS_USB_EVENT_POLL=y
+CONFIG_USB_HOST_ETHER=y
 CONFIG_DM_VIDEO=y
 CONFIG_SYS_WHITE_ON_BLACK=y
 CONFIG_CONSOLE_SCROLL_LINES=10