ldlinux: Always update ConfigName when opening a config file

ldlinux: Always update ConfigName when opening a config file

With the introduction of commit aa7dd29db684 ("ldlinux: Pass config
filename as argv[1] to ldlinux.c32") we stopped using ConfigName to
store the filename of the config file we intend to open in execute()
since it interfered with the filesystem-specific open_config()
implementations. But a side-effect of this change is that
syslinux_config_file() no longer works if we explicitly pass a
filename to parse_config_file().

The logical place to keep ConfigName in sync is in parse_config_file()
since it's expected that all the filesystem-specific open_config()
functions will fill out ConfigName for us. In the case where the
filename is supplied by the caller, we need to fill it out ourselves.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>