configs: sandbox: add USB_ETHER and GADGET_DOWNLOAD gadget support

This is needed for new gadget binding test.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Herbert Poetzl <herbert@13thfloor.at>
Reviewed-by: Simon Glass <sjg@chromium.org>