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(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