bootstd: usb: Add a bootdev driver

Add a bootdev driver for USB host. It can use the distro boot mechanism to
locate a file, or any other available bootmeth.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/usb/host/Makefile b/drivers/usb/host/Makefile
index a4472da..7785b37 100644
--- a/drivers/usb/host/Makefile
+++ b/drivers/usb/host/Makefile
@@ -8,6 +8,10 @@
 obj-$(CONFIG_SANDBOX) += usb-sandbox.o
 endif
 
+ifdef CONFIG_$(SPL_TPL_)USB_STORAGE
+obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += usb_bootdev.o
+endif
+
 # ohci
 obj-$(CONFIG_USB_OHCI_NEW) += ohci-hcd.o
 obj-$(CONFIG_USB_ATMEL) += ohci-at91.o