core: merge the configfile-searching code, isolinux generic names

core: merge the configfile-searching code, isolinux generic names

Merge the config file searching code between syslinux and isolinux,
and make isolinux also recognize the generic names "syslinux" and
"syslinux.cfg".  Thus, isolinux now searches for configuration files
in the following order:

	/boot/isolinux/isolinux.cfg
	/boot/isolinux/syslinux.cfg
	/isolinux/isolinux.cfg
	/isolinux/syslinux.cfg
	/boot/syslinux/isolinux.cfg
	/boot/syslinux/syslinux.cfg
	/syslinux/isolinux.cfg
	/syslinux/syslinux.cfg
	/isolinux.cfg
	/syslinux.cfg

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>