[aboot] add a method for stopping fastboot when usb is unplugged during RX.

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>