helper/command: override target only on target prefixed cmds

helper/command: override target only on target prefixed cmds

In current code the current target is overridden whenever
jim_handler_data is not NULL. This happens not only with target
prefixed commands, but also with cti, dap and swo/tpiu prefixed
commands.
While this is not causing any run-time issue, by now, the
behaviour is tricky and makes the code cryptic.

Add a specific field to struct command for the target override so
the content of jim_handler_data can be restricted to command
specific data only (today only cti, dap and swo/tpiu).

Extend the API register_commands() to specify the presence of
either the command data or the override target.

The new API makes obsolete calling command_set_handler_data() to
set jim_handler_data, so remove it.

Change-Id: Icc323faf754b0546a72208f90abd9e68ff2ef52f
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5667
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>