Initial support for booting from USB drives.

This patch adds initial support for USB Mass Storage Controllers.
This includes support for bulk transfers on UHCI controllers.
Code to detect a USB MSC device is added, and wrappers for sending
    "cdb" block commands over USB are added.
The scsi "inquiry" command is also added.
diff --git a/src/usb-msc.h b/src/usb-msc.h
new file mode 100644
index 0000000..4bad91f
--- /dev/null
+++ b/src/usb-msc.h
@@ -0,0 +1,25 @@
+#ifndef __USB_MSC_H
+#define __USB_MSC_H
+// usb-msc.c
+struct disk_op_s;
+int usb_cmd_data(struct disk_op_s *op, void *cdbcmd, u16 blocksize);
+struct usb_interface_descriptor;
+int usb_msc_init(u32 endp, struct usb_interface_descriptor *iface, int imax);
+int process_usb_op(struct disk_op_s *op);
+struct drive_s;
+void describe_usb(struct drive_s *drive_g);
+ * MSC flags
+ ****************************************************************/
+#define US_SC_SCSI      0x06
+#define US_PR_BULK      0x50
+#define USB_MSC_TYPE_DISK  0x00
+#define USB_MSC_TYPE_CDROM 0x05
+#endif // ush-msc.h