Adding --rbb-port=PORT parameter to the Emulator (#1341)

* Adding --rbb-port=PORT parameter to the Emulator

This is added to allow defining a port for the JTAG Remote Bit Bang, instead of affecting one randomly.
remote_bitbang_t(port); instead of remote_bitbang_t(0);
If no port is selected default value is 0, then the port is chosen randomly.
Tested with OpenOCD + GDB

https://github.com/freechipsproject/rocket-chip/issues/1339

* changed port to rbb_port and added -r in help

Emulator still doesn't take -r PORT in consideration, but it does with --rbb-port=PORT, did I miss something ?

* adding short -r=PORT parameter