target: fix display halt message logic

target: fix display halt message logic

If a target is run from gdb and then stopped from OpenOCD telnet interface,
halt does not show message with status and PC registers.

While on it rename 'display' to 'verbose_halt_msg' and use bool type
instead of int.

Change-Id: Ibe6589015b302e0be97258b06938c297745436a5
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4475
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>