[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>

app/aboot/fastboot.c

@@ -326,6 +326,8 @@ static void fastboot_notify(struct udc_gadget *gadget, unsigned event)
 {
 	if (event == UDC_EVENT_ONLINE) {
 		event_signal(&usb_online, 0);
+	} else if (event == UDC_EVENT_OFFLINE) {
+		event_unsignal(&usb_online);
 	}
 }