ARM ETM/ETB/trace: help/usage updates

Provide helptext which was sometimes missing; update some of it
to be more accurate.

Usage syntax messages have the same EBNF as the User's Guide;
no angle brackets in either place.

Don't use "&function"; functions are like arrays, their address
is their name.  Shrink some overlong lines, remove some empties.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

src/target/etm_dummy.c

@@ -61,9 +61,9 @@ COMMAND_HANDLER(handle_etm_dummy_config_command)
 static const struct command_registration etm_dummy_config_command_handlers[] = {
 	{
 		.name = "config",
-		.handler = &handle_etm_dummy_config_command,
+		.handler = handle_etm_dummy_config_command,
 		.mode = COMMAND_CONFIG,
-		.usage = "<target>",
+		.usage = "target",
 	},
 	COMMAND_REGISTRATION_DONE
 };