command_registration: add empty usage field to chained commands

Chained command require a subcommand as first argument. The usage
field for chained commands is not really important because the
"help" command will list all the subcommands with their respective
usage.

Add a empty usage field on all chained command.
The command "jlink config" can be either followed by a subcommand
or used alone, so use a dedicated usage string.

Change-Id: I43c3f8a766f96a9bdab4e709e3c90713be41fcef
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5017
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/flash/nand/mxc.c

@@ -193,7 +193,8 @@ static const struct command_registration mxc_nand_command_handler[] = {
 		.name = "mxc",
 		.mode = COMMAND_ANY,
 		.help = "MXC NAND flash controller commands",
-		.chain = mxc_sub_command_handlers
+		.chain = mxc_sub_command_handlers,
+		.usage = "",
 	},
 	COMMAND_REGISTRATION_DONE
 };

src/flash/nor/faux.c

@@ -109,6 +109,7 @@ static const struct command_registration faux_command_handlers[] = {
 		.mode = COMMAND_ANY,
 		.help = "faux flash command group",
 		.chain = hello_command_handlers,
+		.usage = "",
 	},
 	COMMAND_REGISTRATION_DONE
 };

src/flash/nor/max32xxx.c

@@ -977,6 +977,7 @@ static const struct command_registration max32xxx_command_handlers[] = {
 		.mode = COMMAND_EXEC,
 		.help = "max32xxx flash command group",
 		.chain = max32xxx_exec_command_handlers,
+		.usage = "",
 	},
 	COMMAND_REGISTRATION_DONE
 };

src/flash/nor/tcl.c

@@ -1260,6 +1260,7 @@ static const struct command_registration flash_command_handlers[] = {
 		.mode = COMMAND_ANY,
 		.help = "NOR flash command group",
 		.chain = flash_config_command_handlers,
+		.usage = "",
 	},
 	COMMAND_REGISTRATION_DONE
 };