Add a signal method to stop fastboot thread when usb cable is plugging out durning firmware download. Signed-off-by: Macpaul Lin <macpaul@gmail.com> Cc: Amol Jadi <amoljadi@codeaurora.org> Cc: Casey Piper <cpiper@codeaurora.org> Cc: Travis Geiselbrecht <geist@foobox.com>