"auto-bsl" through GPIO support for cc2538-bsl.py

[omerk]

This board has solder jumpers for BSL trigger pin (DIO_15) and RESET:

cc2538-bsl.py currently supports toggling the BSL/RESET pins through the use of DTR and RTS signals on the serial port but it would be great to add support for a second mechanism that uses the RPi GPIO to achieve the same behaviour. Perhaps through a new flag introduced, with the BSL and RESET pins that can be passed as arguments (defaults to pins used here?).