[usb] Add basic support for USB hubs

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Author: mcb30

src/Makefile

@@ -83,6 +83,7 @@ SRCDIRS		+= drivers/block
 SRCDIRS		+= drivers/nvs
 SRCDIRS		+= drivers/bitbash
 SRCDIRS		+= drivers/infiniband
+SRCDIRS		+= drivers/usb
 SRCDIRS		+= interface/pxe interface/efi interface/smbios
 SRCDIRS		+= interface/bofm
 SRCDIRS		+= interface/xen

src/drivers/bus/usb.c

@@ -1646,3 +1646,6 @@ struct usb_port * usb_root_hub_port ( struct usb_device *usb ) {
 
 	return usb->port;
 }
+
+/* Drag in hub driver */
+REQUIRE_OBJECT ( usbhub );