ldlinux: fix INCLUDE regression

ldlinux: fix INCLUDE regression

We don't want to overwrite 'ConfigName' when parsing a config file via
the INCLUDE directive, which commit 5447ef821 ("ldlinux: Always update
ConfigName when opening a config file") failed to take into account. In
the INCLUDE case we're only parsing config fragments, and not a main
config file.

Rename parse_one_config() to parse_main_config() to more accurately
reflect when it should be invoked (i.e. not for INCLUDE).

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