target: use correct target in target-prefixed commands and event handlers

This change contains an alternative to Matthias Welwarsky's #4130
(target-prefixed commands) and to #4293 (event handlers).

get_current_target() must retrieve the target associated to the current
command. If no target associated, the current target of the command
context is used as a fallback.

Many Tcl event handlers work with the current target as if it were
the target issuing the event.

current_target in command_context is a number and has to be converted
to a pointer in every get_current_target() call.

The solution:
- Replace current_target in command_context by a target pointer
- Add another target pointer current_target_override
- get_current_target() returns current_target_override if set, otherwise
	current_target
- Save, set and restore current_target_override to the current prefix
	in run_command()
- Save, set and restore current_target_override to the event invoking
	target in target_handle_event()

While on it use calloc when allocating a new command_context.

Change-Id: I9a82102e94dcac063743834a1d28da861b2e74ea
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Suggested-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4295
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>

src/helper/command.h

@@ -49,7 +49,15 @@ struct command_context {
 	Jim_Interp *interp;
 	enum command_mode mode;
 	struct command *commands;
-	int current_target;
+	struct target *current_target;
+		/* The target set by 'targets xx' command or the latest created */
+	struct target *current_target_override;
+		/* If set overrides current_target
+		 * It happens during processing of
+		 *	1) a target prefixed command
+		 *	2) an event handler
+		 * Pay attention to reentrancy when setting override.
+		 */
 	command_output_handler_t output_handler;
 	void *output_handler_priv;
 };
@@ -168,6 +176,11 @@ struct command {
 	command_handler_t handler;
 	Jim_CmdProc *jim_handler;
 	void *jim_handler_data;
+		/* Currently used only for target of target-prefixed cmd.
+		 * Native OpenOCD commands use jim_handler_data exclusively
+		 * as a target override.
+		 * Jim handlers outside of target cmd tree can use
+		 * jim_handler_data for any handler specific data */
 	enum command_mode mode;
 	struct command *next;
 };