dm: usb: sandbox: Add an emulator for USB flash devices

This emulator supports USB enumeration and allows a local file to be provided
as the contents of the emulated flash stick. U-Boot can then use the file as
it would a normal device, with all access passing through the usb_stor layer
and the USB stack.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>
diff --git a/drivers/usb/emul/Makefile b/drivers/usb/emul/Makefile
index f75bbd8..1d5acce 100644
--- a/drivers/usb/emul/Makefile
+++ b/drivers/usb/emul/Makefile
@@ -5,4 +5,5 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
+obj-$(CONFIG_USB_EMUL) += sandbox_flash.o
 obj-$(CONFIG_USB_EMUL) += usb-emul-uclass.o