Proper reset control for stlink_reset

This patch asserts NRST to the target CPU prior to performing the STLINK_DEBUG_RESETSYS command.

The purpose of this patch is to remove the need for me to hit the reset button timed exactly with starting any of the st-utils to gain control of the target processor.  It is also capable of bringing the processor back to life out of deep sleep mode.

I've tested this on the f0 discovery board and a custom board using the F0 tiny model, and on the F4 Discovery.