Wait for device to disappear after reboot-bootloader.

(Linux only for now) With fastboot reading serial numbers from sysfs, it had
become possible for a fastboot command issued immediately after rebooting
the bootloader to fail, because sysfs still thought the device was online.
To prevent this, after reboot-bootloader we wait for the device to disconnect.

Also made usb_read and usb_write fail immediately if the descriptor has been
closed; this prevents an incorrect error message ("Bad file descriptor")
when errors from fb_getvar are ignored (e.g., by fb_format_supported).

Also removed unused fd param from filter_usb_device, and simplified logic
in usb_write by using do/while instead of a special case for len == 0.

Change-Id: I799b857eab411fd8ad25f5777fc61c685152ea86
diff --git a/fastboot/usb_windows.c b/fastboot/usb_windows.c
index 7aa36b2..07f7be2 100644
--- a/fastboot/usb_windows.c
+++ b/fastboot/usb_windows.c
@@ -269,6 +269,11 @@
     return 0;
 }
 
+int usb_wait_for_disconnect(usb_handle *usb) {
+    /* TODO: Punt for now */
+    return 0;
+}
+
 int recognized_device(usb_handle* handle, ifc_match_func callback) {
     struct usb_ifc_info info;
     USB_DEVICE_DESCRIPTOR device_desc;