target: OpenOCD fails with assert during running "reset" command

OpenOCD fails in the presence of inactive/unresponsive cores

I faced with case when inactive core returns 0 while reading dtmcontrol.
This leads to failure on assert: "addrbits != 0" in "dbus_scan".

Also change "read_bits","poll_target" funcs to avoid a lot lines in logs

Change-Id: If852126755317789602b7372c5c5732183fff6c5
Signed-off-by: Kirill Radkin <kirill.radkin@syntacore.com>