Added support for enabling just SWD without JTAG

This pull request adds the option USE_SWD, which enables SWD debugging, but keeps JTAG disabled.
This is useful when the JTAG pins are already in use.

To compile with SWD enabled, JTAG disabled, run:

    make clean all USE_SWD=y

The ST-Link V2 works in combination with ST-util with a very simple circuit, directly connecting SWDIO, CLK, VDD, GND and RESET. I'll post the details on the forum:
https://community.spark.io/t/working-jtag-programming-and-debugging-from-netbeans/7679