Fix SONOFF chip reset after flashing

There's nothing too wacky about the connections on the SONOFF dongle,
it's just that:

    DTR = RESET_N (active low)
    RTS = !DIO_15 (inverted with transistor)

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

@@ -219,7 +219,7 @@ def invoke_bootloader(self, dtr_active_high=False, inverted=False, sonoff_usb=Fa
         # DTR: connected to the bootloader pin
         # RTS: connected to !RESET
         # If inverted is True, pin connections are the other way round
-        if inverted:
+        if inverted or sonoff_usb:
             set_bootloader_pin = self.sp.setRTS
             set_reset_pin = self.sp.setDTR
         else:
@@ -232,15 +232,16 @@ def invoke_bootloader(self, dtr_active_high=False, inverted=False, sonoff_usb=Fa
             # ITead Sonoff Zigbee 3.0 USB Dongle. This dongle has an odd
             # connection between RTS DTR and reset and IO15 (imply gate):
             # DTR  RTS  |  RST  IO15
-            # 1     1   |   1    1
+            # 1     1   |   0    0
             # 0     0   |   1    1
             # 1     0   |   0    1
             # 0     1   |   1    0
-            set_bootloader_pin(0)
-            set_reset_pin(1)
-
-            set_bootloader_pin(1)
-            set_reset_pin(0)
+            set_reset_pin(0)        # Enter reset
+            set_bootloader_pin(1)   # Necessary for some reason
+            set_bootloader_pin(0)   # Assert bootloader pin
+            set_reset_pin(1)        # Exit reset
+            time.sleep(0.002)       # Wait for chip to enter bootloader
+            set_bootloader_pin(1)   # De-assert bootloader pin
         else:
             set_bootloader_pin(1 if not dtr_active_high else 0)
             set_reset_pin(0)