gdb server: Add back support to disable the gdb server via the gdb_port config

As per the documentation, used "disabled" as the value to disable, as this
is the same value to disable the telnet and tcl server.

Change-Id: Idc4a8580098ec1107dcc6e1f59e817ecdebc38ac
Signed-off-by: Sandeep Mistry <s.mistry@arduino.cc>
Reviewed-on: http://openocd.zylin.com/3175
Tested-by: jenkins
Reviewed-by: Cristian Maglie
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

doc/openocd.texi

@@ -2109,7 +2109,7 @@ communicate via pipes(stdin/out or named pipes). The name
 the normal use cases.
 
 No arguments reports GDB port. "pipe" means listen to stdin
-output to stdout, an integer is base port number, "disable"
+output to stdout, an integer is base port number, "disabled"
 disables the gdb server.
 
 When using "pipe", also use log_output to redirect the log

src/server/gdb_server.c

@@ -2921,6 +2921,11 @@ static int gdb_target_start(struct target *target, const char *port)
 
 static int gdb_target_add_one(struct target *target)
 {
+	if (strcmp(gdb_port, "disabled") == 0) {
+		LOG_INFO("gdb port disabled");
+		return ERROR_OK;
+	}
+
 	/*  one gdb instance per smp list */
 	if ((target->smp) && (target->gdb_service))
 		return ERROR_OK;
@@ -3114,7 +3119,7 @@ static const struct command_registration gdb_command_handlers[] = {
 			"server listens for the next port number after the "
 			"base port number specified. "
 			"No arguments reports GDB port. \"pipe\" means listen to stdin "
-			"output to stdout, an integer is base port number, \"disable\" disables "
+			"output to stdout, an integer is base port number, \"disabled\" disables "
 			"port. Any other string is are interpreted as named pipe to listen to. "
 			"Output pipe is the same name as input pipe, but with 'o' appended.",
 		.usage = "[port_num]",