[parseopt] Refer to online documentation for command help

The online documentation (e.g. http://ipxe.org/cmd/ifopen), though not
yet complete, is far more comprehensive than could be provided within
the iPXE binary.  Save around 200 bytes (compressed) by removing the
command descriptions from the interactive help, and instead referring
users directly to the web page describing the relevant command.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/hci/commands/pxe_cmd.c

@@ -33,7 +33,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 /** "startpxe" command descriptor */
 static struct command_descriptor startpxe_cmd =
 	COMMAND_DESC ( struct ifcommon_options, ifcommon_opts, 0, MAX_ARGUMENTS,
-		       "[<interface>]", "" );
+		       "[<interface>]" );
 
 /**
  * "startpxe" payload
@@ -68,8 +68,7 @@ static struct option_descriptor stoppxe_opts[] = {};
 
 /** "stoppxe" command descriptor */
 static struct command_descriptor stoppxe_cmd =
-	COMMAND_DESC ( struct stoppxe_options, stoppxe_opts, 0, 0,
-		       "", "" );
+	COMMAND_DESC ( struct stoppxe_options, stoppxe_opts, 0, 0, "" );
 
 /**
  * The "stoppxe" command

src/arch/i386/hci/commands/reboot_cmd.c

@@ -36,7 +36,7 @@ static struct option_descriptor reboot_opts[] = {};
 
 /** "reboot" command descriptor */
 static struct command_descriptor reboot_cmd =
-	COMMAND_DESC ( struct reboot_options, reboot_opts, 0, 0, "", "" );
+	COMMAND_DESC ( struct reboot_options, reboot_opts, 0, 0, "" );
 
 /**
  * The "reboot" command