made openocd always use SWD and added small note about gdb debugging files

openocd tried "Info : auto-selecting first available session transport "jtag". To override use 'transport select <transport>'." using a JLink debugger. Overriding it with SWD is what I did. With the CMSIS debugger openocd you get lucky, by Info : auto-selecting first available session transport "swd".. Hope forcing it always to SWD will make it easier for others using different debuggers.