helper/command: send command output only to the right server

helper/command: send command output only to the right server

When opening multiple telnet connections, the command output on
one console is broadcast to all the other console.

Send the command output only to the output handler that has been
set for that context.
For the moment, still broadcast the errors.

Note that contexts which doesn't explicitly set the output handler
(e.g. gdb_server), will inherit the global output handler
configuration_output_handler() that will broadcast the output.

Change-Id: I1f2d6e6a3bbf730f5fa77246730fce1445742a1e
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5093
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>