target: Add a debug_reason command

This allows programmatically determining the debug reason.

Change-Id: I0c3e85cebb6dc28fc0fc212beca84a484ac654a5
Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7952
Reviewed-by: Jan Matyas <jan.matyas@codasip.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins

doc/openocd.texi

@@ -5296,6 +5296,18 @@ Displays the current target state:
 (Also, @pxref{eventpolling,,Event Polling}.)
 @end deffn
 
+@deffn {Command} {$target_name debug_reason}
+Displays the current debug reason:
+@code{debug-request},
+@code{breakpoint},
+@code{watchpoint},
+@code{watchpoint-and-breakpoint},
+@code{single-step},
+@code{target-not-halted},
+@code{program-exit},
+@code{exception-catch} or @code{undefined}.
+@end deffn
+
 @deffn {Command} {$target_name eventlist}
 Displays a table listing all event handlers
 currently associated with this target.