gdb_server: support disabling server

Although the documentation suggested this worked, and it is implemented
for tcl_port and telnet_port, the directive was not recognized for
gdb_port.

Change-Id: I38d95ee879ec3f6d551603b7313749a21e0e498e
Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Reviewed-on: http://openocd.zylin.com/3637
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>

src/server/gdb_server.c

@@ -2944,6 +2944,11 @@ static int gdb_target_add_one(struct target *target)
 
 int gdb_target_add_all(struct target *target)
 {
+	if (strcmp(gdb_port, "disabled") == 0) {
+		LOG_INFO("gdb server disabled");
+		return ERROR_OK;
+	}
+
 	if (NULL == target) {
 		LOG_WARNING("gdb services need one or more targets defined");
 		return ERROR_OK;