[syslinux] menu: alternative config file

[Sebastian Herbszt]

The menu system allows one to specify an alternative config file and it's also possible to
specify more than one file. menu.txt got
"If you specify more than one file, they will all be read, in the order specified."

What is the desired failure mode if one of those files doesn't exist?

com32/menu/menumain.c calls parse_configs() which itself calls parse_one_config (readconfig.c).
parse_one_config() returns -1 if the specified config file is not available, but the return code is not
checked in parse_configs(). The current failure mode is therefore "ignore". If this is by design it might
be worth mentioning it in menu.txt.

If no config file is found the menu system clears the screen, but doesn't display anything until a key
is pressed (no "grid", no "press tab key" message, something's broken here?).

- Sebastian