Fix SONOFF chip reset after flashing

The chip will return to the bootloader after cmdReset if the bootloader
pin is not de-asserted after entering the bootloader. To fix this, this
patch de-asserts the bootloader pin after a delay.

cc2538-bsl.py

@@ -241,6 +241,10 @@ def invoke_bootloader(self, dtr_active_high=False, inverted=False, sonoff_usb=Fa
 
             set_bootloader_pin(1)
             set_reset_pin(0)
+
+            # wait to enter bootloader, then deassert bootloader pin
+            time.sleep(0.002)
+            set_reset_pin(1)
         else:
             set_bootloader_pin(1 if not dtr_active_high else 0)
             set_reset_pin(0)