rtt: default the ID to "SEGGER RTT"

Instead of making people type this in all the time, just default to
"SEGGER RTT" so more things work out of the box.

Change-Id: I147142cf0a755e635d3f66e047be2eb5049cf511
Signed-off-by: Karl Palsson <karl.palsson@marel.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8354
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

doc/openocd.texi

@@ -9541,11 +9541,12 @@ Channels are exposed via raw TCP/IP connections. One or more RTT servers can be
 assigned to each channel to make them accessible to an unlimited number
 of TCP/IP connections.
 
-@deffn {Command} {rtt setup} address size ID
+@deffn {Command} {rtt setup} address size [ID]
 Configure RTT for the currently selected target.
 Once RTT is started, OpenOCD searches for a control block with the
 identifier @var{ID} starting at the memory address @var{address} within the next
 @var{size} bytes.
+ID defaults to the string "SEGGER RTT"
 @end deffn
 
 @deffn {Command} {rtt start}
@@ -9588,7 +9589,7 @@ on the target device.
 @example
 resume
 
-rtt setup 0x20000000 2048 "SEGGER RTT"
+rtt setup 0x20000000 2048
 rtt start
 
 rtt server start 9090 0

src/rtt/tcl.c

@@ -19,8 +19,14 @@ COMMAND_HANDLER(handle_rtt_setup_command)
 {
 	struct rtt_source source;
 
-	if (CMD_ARGC != 3)
+	const char *DEFAULT_ID = "SEGGER RTT";
+	const char *selected_id;
+	if (CMD_ARGC < 2 || CMD_ARGC > 3)
 		return ERROR_COMMAND_SYNTAX_ERROR;
+	if (CMD_ARGC == 2)
+		selected_id = DEFAULT_ID;
+	else
+		selected_id = CMD_ARGV[2];
 
 	source.find_cb = &target_rtt_find_control_block;
 	source.read_cb = &target_rtt_read_control_block;
@@ -38,7 +44,7 @@ COMMAND_HANDLER(handle_rtt_setup_command)
 
 	rtt_register_source(source, get_current_target(CMD_CTX));
 
-	if (rtt_setup(address, size, CMD_ARGV[2]) != ERROR_OK)
+	if (rtt_setup(address, size, selected_id) != ERROR_OK)
 		return ERROR_FAIL;
 
 	return ERROR_OK;
@@ -218,7 +224,7 @@ static const struct command_registration rtt_subcommand_handlers[] = {
 		.handler = handle_rtt_setup_command,
 		.mode = COMMAND_ANY,
 		.help = "setup RTT",
-		.usage = "<address> <size> <ID>"
+		.usage = "<address> <size> [ID]"
 	},
 	{
 		.name = "start",