Minor ETB and ETM bugfixes and doc updates

Minor ETB and ETM bugfixes and doc updates

 - ETB
    * report _actual_ hardware status, not just expected status
    * add a missing diagnostic on a potential ETB setup error
    * prefix any diagnostics with "ETB"
 - ETM
    * make "etm status" show ETM hardware status too, instead of
      just traceport status (which previously was fake, sigh)
 - Docs
    * flesh out "etm tracemode" docs a bit
    * clarify "etm status" ... previously it was traceport status
    * explain "etm trigger_percent" as a *traceport* option

ETM+ETB tracing still isn't behaving, but now I can see that part of
the reason is that the ETB turns itself off almost immediately after
being enabled, and before collecting any data.

git-svn-id: svn://svn.berlios.de/openocd/trunk@2790 b42882b7-edfa-0310-969c-e2dbd0fdcd60