Fixes to make openocd work well with PX4

Per discussion with the px4devs, here's a set of changes needed to make openocd debugging work well with the PX4.  I've also updated the wiki:

https://pixhawk.ethz.ch/px4/dev/jtag/openocd
and
https://pixhawk.ethz.ch/px4/dev/jtag/gdb